I've been collecting various D&D editions. Recently I decided to start looking at 3.0. I played very little during this era. I feel like I missed out seeing all the material that was released for this edition (3e/3.5). I just bought a first printing of the 3.0 PHB, the book is in great condition. I noticed in the very back of the book there is a DMG/Monster Manual primer. I never knew this was included in the phbs. I owned a copy back when it was current but I don't believe it was a first print run. I don't recall seeing this small section in my copy.
Yeah, this was specifically in the first print run, since the DMG and MM were released as subsequent monthly intervals. The idea was basically "here's some basics to tide you over until the real book is released."
Three Forgotten Reams Books from the end of 2E are great, and actually not too many mechanics to translate iirc. Lands of Intrigue, Empires of the Shining Sea, and Sea of Fallen Stars. All by Dale Donovan and Steve Schend. Those 2 guys had a great FR run 95-2000
Lands of Intrigue is on my hate list, though that's probably more due to novel-based metaplot advancement than any inherent quality issues. The 2e FR box described Tethyr as a country torn by civil war, where the royal family had been assassinated ~20 years ago and the nobles had been fighting with one another since then. I thought that seemed like a good place to run a campaign, so when I see there's a box set out that's supposed to cover Tethyr (and Amn, but that's less important) I buy it... and is presented with a Tethyr where The Rightful King (who was in hiding as Elminster's scribe) and his Amazeballs Queen has returned and pacified all the nobles and instituted just rule where everyone is happy except the few bad nobles who got exiled. I think that's also where my distaste for metaplot emerged.
Other than that... most of the Eberron books are amazing, if that's your jam. The campaign setting itself of course, but some of the other books are also stellar.
Sharn: City of Towers has a fantastic approach to making a city sourcebook: instead of trying to map out a gigantic city, it focuses on describing various districts and what you can find there. There's also a really meaty chapter on how various things work in the city. You want entertainment? That's mostly found in these districts, with more of X over here and more of Y over here. Oh, you meant "entertainment"? That's over there. Interested in scholarship? Morgrave University's the place to be, even if they have a bit of a shady rep compared to other universities. Stuff like that. It does have one flaw in the modern day and that's how the district system relies on a DMG web enhancement which I don't think is publicly available anymore (though it can probably be found with some googling – the name is "building a city" so googling that plus "web enhancement" should probably find some place where it's archived).
Player's Guide to Eberron takes a similar approach to parts of the Sharn book in that it has a bunch of short sections (2-4 pages for the most part) on a vast variety of things in the setting. The Last War and its effects, Magical Traditions, Society & Status, The Dark Six, Dragonshards, The Blood of Vol... a whole lot of things that benefit from some more info than what's in the ECS but maybe doesn't need a full sourcebook.
After that it kind of depends on where one's interests lie. Most Eberron material is good (though Forge of War perhaps less so). I'll give a final shoutout to Faiths of Eberron because of its focus on actual
religion instead of gods. The gods of Eberron do not directly interfere with the world, at least not in measurable ways (though their followers believe the gods aid them in everyday life – when the hunter comes home with a deer to cook, they believe Balinor helped their arrow strike true, and when the soldier is facing an enemy charge they pray to Dol Dorn for courage), so you won't find any stats for gods or avatars here. Instead you'll learn what the people of Khorvaire actually believe, the differences and similarities between belief in the Sovereign Host and the Dark Six, and how they often intermingle, why people would follow an undead-focused faith like the Blood of Vol, and so on.
Looking at more generic 3e books:
Tome of Battle provides some martial classes and a system of martial pseudo-magic: special maneuvers these classes can use that make their fighting a little more interesting than determining how much power attack to use on a particular target. In many ways, this was a predecessor to some of the good stuff in 4e.
Expanded Psionics Handbook is probably the mechanically best implementation of psionics rules in D&D, if you like that sort of thing.
Magic Item Compendium both provides some patches to the regular rules about magic items (most notably, letting you combine a "bonus to important game stat" item with a "lets you do a neat thing" item without paying a surcharge for combining effects in a single item slot), and provides a fairly large number of low-level magic items that are otherwise missing in 3e. If you're planning to actually run the game it's an excellent book to have, but it's not a particularly
exciting book.