This book/pdf is 82 pages, 1 page front cover (in the dead-tree version blank inside), 1 page editorial/ToC, 1 page advertisement, 1 page SRD, 1 page back cover. That leaves 77 pages of content.

Disclaimer: I was a patron of this project but due to real life, didn't contribute that much to this project.

The pdf is full-color with some b/w artwork here and there, while the dead-tree version is b/w.

The book kicks off with an informative 2-page introduction to the creation of the aboleth as a monster by David "Zeb" Cook, its original creator.

Then, we get a chapter of 8 pages on lost cities in real world myths and legends, namely Atlantis (D'uh), Lemuria and Mu. This chapter also details a sample lost city, Ankeshel and the city of archeaologists, tomb-raiders etc. that spawned from it, Cassadega. 1 page is taken up by an absolutely gorgeous, full-color map of the twin cities. While I didn't care too much for the information on real world empires (most of which I already did know), I loved the Ankeshel-write-up and think that most people may benefit from this chapter.

Chapter 2 deals with pelagic Characters, i.e. characters that are especially competent/adapted to frequent underwater expeditions. (14 pages)
Within, we get a new race, the Maerean (Half-Merfolk), who gets +2 to one ability score, low-light vision, multitalented, keen senses, count as both merfolk and human and are capable of living both in water and on land. This race left me under-whelmed, probably because of my utter hatred of "environmental races", I.e. races that negate environmental perils. HOWEVER: This race does not really negate cold or pressure, so it does not wholly fall into this despised category. We are, thankfully, spared aquatic dwarves and gnomes and the like and instead get some cool and useful bits and pieces: All the classes get alternative class features for pelagic characters. Barbarians get rapid swimming as well as 7 new rage powers. Bards get 3 new bardic performances. Clerics get 5 new domains (Deep, Glyph, Fish, Ocean, Sea Monster). Druids get advice on aquatic animal companions and rapid swimming. Fighters get underwater armor training, a bonus to swim checks and aquatic weapon training. Monks get a new ability for their Ki pool, fast swimming, can prevent animals and magical beasts from attacking them, get more breath, gain "Ki swimming", water walk and later can even turn their body to water. Paladins can use lay on hands to replenish breath and grant weapons the "Wavesplitting" quality as well as get an aquatic mount. Rangers get access to the net-and-trident combat style, can move easier through difficult terrain, get better at killing undersea foes and scent. Rogues get 6 new Rogue Talents and 3 new advanced talents. Sorcerors get3 new bloodlines: Aboleth, Vril and Ocean. The Aboleth and Vril bloodlines count amongst the best bloodlines I've read so far. Wizards get two cool new schools, the glyph school and water school. Both arcane casters can profit from the new familiars presented later in the book.

We also get 25 new feats:

Aboleth Spawn - Use acid splash as a spell-like ability
Antiquarian Tinkerer -Use Disable Device to operate ancient technology
Aqualung - Hold breath for extended duration
Artifact Hunter - +2 bonus on Appraise and Knowledge (history) checks
Bioluminescence - Use hypnotism as a spell-like ability
Born to the Water - Increased swim speed; +2 to Con checks
Call Leviathan - Add template to summoned aquatic creature
Castaway - +2 bonus on Knowledge (nature) and Survival checks
Cranial Manipulation - Increase number of Hit Dice affected by enchantment spell
Deep Diver - Decrease damage from cold and pressure
Depth Affinity - Immune to cold/pressure damage
Elemental Connection - Spells cast in water are more powerful
Fisherman’s Eye - +4 bonus on Perception checks underwater
Fish Skin - Heal at double rate when in water
Glyph Familiar - Your familiar also benefits from personal spells
Glyph Reader - +4 bonus to operate ancient technology and magic
Hydromancer - Add water descriptor to prepared spell
Improved Net Fighter - Control entangle opponent as a move action
Agile Netter - Opponents in net are flat-footed
Ioun Stone Bond - choose ioun stone as your bonded item
Leviathan Slayer - +2 to attack/damage against Large aquatic creatures
Like an Eel — Make Acrobatics checks instead of Swim checks
Long Distance Swimmer - +4 bonus to avoid non-lethal damage when swimming
Merchant Adventurer - +2 bonus on Appraise, Diplomacy and Linguistics checks
Net - +2 to DC of check made while entangled
Pattern Glyph - +1 to DC of pattern spells
Quick Brace- wielding a Brace weapon as immediate or swift action
Quick Fold - net Fold a net as a move action
Rebuke Sea Creatures - Channel energy can be used to make aquatic creatures flee
Roil -Channel energy can be used to create area of turbulent water
Rigging Monkey - +2 bonus on Acrobatics and Climb checks
Sea Dog - +2 bonus on Knowledge (geography) and Profession (sailor) checks
Strong Swimmer - Bonus to Swim speed.
Swim-by Attack - attack while swimming
Underwater Caster - +4 on concentration checks to cast spells underwater
Unhampered Casting - Bonus to cast spells underwater without components
Vril Heritage - Use mage hand as a spell-like ability
Water Fighter - No penalty to attack with selected weapon underwater
Waterstep - Briefly walk on water
Waterwise - +2 bonus to Knowledge (nature) and Perception checks
Wave Fighter - Ignore underwater fighting penalties for one weapon group
Will to Live - Use Will save instead of Constitution check to avoid drowning
Metamagic
Englyph Spell - Add glyph spell to other spell
Shielded Spell - No concentration check to cast spell underwater


None of these feats felt over-powered and all have their place and contain their own little stories/information on the characters and even better, come with a little bit of cool fluff at the beginning - Nice! Also: I'd probably take Aqualung just because the name is a nice nod to Jethro Tull. ;P I also liked the new ranger fighting style that could also be used in gladiatorial settings. However, several typos and minor editing mistakes (e.g. "Avoiddrowning"[Sic!]) have went by the proofers here, somewhat impeding my enjoyment of the chapter.

Chapter 3 deals with new aquatic equipment and technology of lost cities. (12 pages). Oh boy. THIS CHAPTER ROCKS!! Big time. You get new armor, weapons and equipment, from seashell armors over diving suits to several new weapons. And you get an awesome smattering of new special & alchemical substances like a stick with an explosive coral at the top or shoggoth polyps. Each of these little items spawned some great idea in my mind and made me want to implement them right now. They are gold, pure and simple. We also get Orichalcum as a new material for weapons, armor, etc. and new lost technology in the form of Ankeshelian Vril weapons and associated equipment. Oh boy do they succeed at making far-out weapons feeling both like technology and yet utterly alien. Their firearms rocks and don't feel at all like gun-powder weapons and for all of you who don't like rifles and the like: They are powered via special vril batteries and you can easily restrict/ ban access to these in your home-game. This is one of the most concisely-written equipment chapters I've ever read. My only gripe here is: I want more!

The next chapter gives us the obligatory new spells and magic items. (16 pages) While I am usually skeptical towards new spells as a result of 3.5-glut, this chapter manages both to introduce cool new spells that e.g. enable you to cry at lower levels with the risk of losing your sight as well as spells that might fuel adventure plots or flat-out help you survive in the water, even at lower levels. Then came the magic items. People who frequently read my reviews know that I'm quite fiercely opposed to magic item-glut, too, and that I abhor the fact that too many generic magic items detract from the wonder one is supposed to feel when encountering them. I' happy to report that these items all have this quality of inventiveness, of sheer coolness, that makes you actually want to introduce them to your campaign, and not only to give your PCs another bonus. One of my favorites is e.g. a dried-up blowfish that circles adventurers and hurts attacks. How cool is that?

The penultimate chapter details the sunken environment (7 pages). It begins with an introduction and advice for GMs to plan underwater campaigns according to level. This section unfortunately features a piece of stock art that feels just out of place. This chapter also details new environmental zones that include information on natural hazards, how e.g. dunes affect movement, the tides, algae etc. - it is guaranteed to enrich your campaigns for years to come and another prime example of extremely concise writing. The hazards alone are plain awesomeness.

The final chapter details new creatures of the deep (17 pages). Unfortunately, this chapter does not kick of with the usual kick-ass full-color artwork, but it does feature original b/w artworks for most monsters. The artworks were ok, but didn't absolutely wow me.

The monsters are:
-Bone Crab (CR 2): Aggressive crabs with hive-mind
-Bore Worm (CR 7): Killer Barnacle-like creature, include a variant
-Cnidari (CR 4): Jelly-fish-like abberration: Disturbing, cool race.
-Coral Drake (CR 8): Chameleon dragon-like creature, has a very cool breath weapon.
-Doldrum (CR 8): Evil Elemental. Felt rather bland to me, as the type of creature has been done x times in Ravenloft
-Drowned Maiden (CR 7): A classic trope in a very nice incarnation.
-3 new Statblocks for new familiars (Cr 1/3): Only one piece of artwork is given. Anthropod familiar stat-block gets 4 alternate creatures, Cephalopod also gets 4 and Fish gets 5. You can now have a squid familiar. How cool is that?
-Fish, Dunkleosteus (CR 7) and Giant Angler Fish (CR 5) share a page and get only one piece of artwork.
-Goblin Shark (CR 3): This shape-changer brings the goblin-brand havoc beneath the waves. And is kinda creepy in the awkward, cute goblin way.
-Isonade (CR 16): A legendary monster that unfortunately only features its fin in the artwork. I would have loved to see the whole creature.
-Wharfling (CR 1/2) and Wharfling Swarm (CR 4): Pilfering, diseased, hairless rats. I like them.

We also get a page of aquatic variants of known monsters, which is a nice way to make the familiar new.

The final pages of the book are taken up with an ecology of the Aboleth, complete with a b/w artwork and 4 Aboleth variants. While I liked the prose and the rules and this definitely is one of the better ecologies, I have to say that this ecology suffers from my frame of reference: "Lords of Madness" and "Secrets of the Mimic" have spoiled me so much with regards to these critters, that I didn't get that much out of this article.

Conclusion:

Sunken Empires is an awesome resource, there is no way around it. If you are even remotely contemplating on adding aquatic elements to your campaign, this is the place to go. It is densely and concisely written, there is a huge wealth of information contained herein. However, the book is not perfect: While the artwork is mostly awesome, there are some stock-art pictures that just feel utterly out of place. Some of the monster-artwork didn't impress me either. Layout is just plain beautiful and serves to enhance the sense of wonder one should feel for the new environment and items. Editing, though, is below the high standard of this book - there are several typos and some other editing glitches that irked me, doubly so due to the quality of the information contained herein. While some of the chapters rank among the best I've ever read, I could have e.g. done without the recap of Atlantis, Mu, Lemuria etc., but I recognize that for some DMs these chapters might be useful.

What I don't understand is this, though: If there is such a beautiful pdf, why is there no option to buy a more expensive copy of this book in full-color? Bringing this to the table and showing of some of the amazing chapter lead-in artworks would certainly enhance the overall experience. My final complain is, that I would have loved to see more, which is more of a compliment than anything else.

While I love the mundane and Ankeshelian equipment as well as the hazards, spells and magic items, some of the other components of the book felt slightly short of this status of genius.
Due to the fact that there are some minor issues with editing and some variations in quality with regards to the information presented herein, I'll settle for a solid 4-star-score and heartily recommend you purchase this very good, but not quite perfect book. Chances are, you won't regret it.