When I was a kid, and first started playing AD&D (1st edition), one of the funnest things about making a magic-user character was rolling for your familiar. You always wanted a pseudo-dragon. Usually you'd end up with something lousy.
While 3rd edition D&D made familiars more balanced (and even more so in 3.5, which this book apparently is based on), it also took the magic out of them, somewhat. This book goes a long way to making familiars anything but ordinary. And it covers familiars about as exhaustively as you can cover them.
It's from Dark Quest games, which I honestly had never heard of, though I do recognize some of the authors and people involved with the company. It's available as a PDF (from RPG Now, like just about all PDFs) or in print form, which I have, which you can order from their website or since it has an ISBN, presumably someplace else in the future.
After some introductory fiction, it starts with a nice section on role-playing the familiar. There's a listing of negative personality traits (most of which actually apply to normal cats, like "narcissistic" and "irritating"), though one section, on phobias, seems to refer to an appendix which does not exist. (This is pretty minor, as it apparently must have been just a list of phobias. If you play Call of Cthulhu, there's a pretty extensive list of them in the rulebook for that).
Still, one tricky part is, who plays the role of the familiar? If the player of the owner/master does it, it can be somewhat hard, since you'll basically be talking to yourself. (Not a new problem, it's always been the same for henchmen).
Besides role playing aspects of the familiar, there's a fairly examination of just what a familiar is. Is it a regular critter given special powers? Some sort of ghost? A tulpa (that is, a figment of the master's mind, only real)? It's up to you, there are several explanations.
There's also rules on how to integrate the familiars from this book into your game easily. Basically, there are two options for the improved or exotic familiars - either the casting of a higher level spell, or requiring the use of a feat, either of which is dependent on the type of improve or exotic familiar.
I favor the feat method, as most familiars give the spell-caster a benefit equal to that of a feat, a skill bonus of some sort usually, so it balances back out.
Just about every possible type of familiar is covered. Elementals and other planar critters, undead, constructs (like the mechanical own from "Clash of the Titans"), birds, reptiles, rodents, vermin (bugs and such, not lawyers), oozes, monstrous humanoids, and "exceptional beasts". By a quick count, over 125 different familiars.
Each chapter on a type of familiar starts with a list of familiars and their benefits, a discussion of the type of creatures (often including special rules and role-playing ideas), then ends with stats for each type of critter.
While D&D has always offered "Planar" creatures as familiars, it was pretty much limited to the Imp or the Quasit. The section on planar creatures fills in the gaps for most of the other planes, both "outer" (the planes related to alignment) and "inner" (elemental planes)
The most fascinating familiar in this section is the "Phenix", which is like a little bird shaped ball of fire (named after the legendary "Phoenix"), that changes colors according to its moods.
Undead familiars range from the fairly mundane zombie or skeleton, to the rather cool "flying skull". (There was one in the computer game, Planescape: Torment). Obviously, these are mostly suited for necromancer types and those wanting to impress goth chicks.
There are a whole lot of choices if you want a bird or avian familiar. From just about every bird associated with a sports team to things like loons and toucans and messenger pigeons. And apparently fictional creatures like the "ghost bat".
The reptile chapter is a bit shorter, but it has things like my favorite, the gila monster, to the komodo dragon, to iguanas and such.
Rodents. Lots and lots of rodents. Mostly cute ones, chipmunks, chinchillas, ferrets, weasels, squirrels, mongeese, minks. But also opossums (possibly nature's saddest creature, though surprisingly human like, especially their hands), rats, lemmings, and mice. (Among others).
Want a spider familiar? The vermin chapter covers them pretty well, as well as cockroaches. Also crickets, dragonflies and snails are included.
I love the exotic animals in the exceptional beast chapter There are choices for animals of all sorts, from a koala bear to a wombat to a pot bellied pig to a penguin. (You would have to resist calling it "Opus", but that is made easy thanks to the painfully unfunny comeback cartoon of the same name.) But there is some chance of someone taking a silly familiar, or one that might not fit all campaigns.
Oddly, there are also aquatic familiars in this chapter, things like a trout or an eel or a nurse shark. I guess these are mostly for aquatic spell casters, like merpeople or tritons.
The monstrous humanoid category is mostly small humanoids, like goblins or kobolds. While I'm not sure I'd use this in my game, I'm not sure it's unprecedented, exactly. In 1st edition AD&D, you could get a Brownie (the small faerie, not the food or girl scout) as a familiar, which definitely was sentient. But they were somewhat magical. Things like kobolds or goblins or even tieflings seem fairly ordinary, at least they are in a fantasy world, and so are basically well, people.
I'm not sure what to make of the ooze familiars. Other than "Creepy!". There's not a lot of options here, just the choice between a "Goozaling", which is a typical ooze, only much smaller, and a "Taffinymph", which is a type of goozaling that has been bred to look and smell like taffy (Much like how in real life, poodles have been bred to resemble cotton candy).
There's a small chapter equipment for familiars, including food (and how much they need a day), and a handful of magic items, like bells and collars for them to wear. This is pretty short, and probably could have been expanded on. For instance, there is a cursed collar the causes the familiar to suffer from fleas, but there is no magic collar to prevent them (which would be very useful. Probably for the master, too, not just the familiar)
The last chapter is sort of a grab bag of stuff, though it's mostly on making a familiar tougher, including giving guidelines for allowing familiars to take classes. It also features rules for allowing spell casting classes other than the Wizard or Sorcerer to have a familiar. There's a single prestige class, the "Magebond", which is for Wizards or Sorcerers who want a really special familiar. There's also a handful of familiar related spells.
Also, in an interesting move, they borrow the "Prestige Race" rules from Bastion Press's Oathbound setting. Basically, in exchange for experience points, a character gets special abilities. I'm somewhat iffy about this for characters (because it messes up the challenge rating of characters), but in familiars it actually works pretty well.
While all in all, it's a very impressive effort, there are some downsides to this book. It's sometimes contradictory. For instance, in the chapter on birds, it says a Seagull gives the caster a +2 charisma bonus on skills used on sea-goers. But in in a later chapter, on exceptional beasts, it says a "Gull" (no sea, but presumably the same bird) gives the master a +3 bonus on Profession (Fishing) skill checks. In the description of the Penguin, it says the master gets a +2 on their saving throw vs. cold effects. But in the chart at the beginning of the chapter, it says the master gets a +3 bonus on sleight of hand checks (Penguins being notorious pickpockets, ya know).
I'm not sure all the familiars are balanced, in terms of the benefits they give the spell caster. For instance, several give the caster a +2 charisma score. That is pretty big for a sorcerer, as charisma is what determines their bonus spells. But only a small handful do that, so it's pretty minor.
While it has a lot of animals, a whole lot, it doesn't have my personal favorite, the kangaroo.Though it does have my favorite bird, the sandpiper. And while it has my favorite baseball team's mascot (the cardinal), it doesn't have my favorite football team's (ram), though it does have their coach (loon, or maybe cuckoo).
The layout is quite good. While there is no index, there is an extremely comprehensive table of contents (3 pages long, with lots of entries per chapter) which makes finding things a breeze. There's almost no white space, which makes it a very good value. While there is a fair amount of "fluff", like introductory fiction at the beginning of each chapter, it's fairly enjoyable and does a good job of setting the tone of each chapter.
The art (by someone called "Ceredwyn") is actually quite good. It's mostly comic in tone, and occasionally disgustingly cute, but that pretty much fits the book's style (and much of the fiction).
If you want a book on familiars, you'd be hard pressed to find a better or more comprehensive one. It's also pretty useful as a monster manual of mundane and small-ish critters. I'm very impressed with this. A-
N.B. Yes, I know it's "Mongooses", but I like "Mongeese" better.