What d20 game products beyond the core rule books do you think are truly really good books that add depth to the system...and if you want to answer this one, which books are your personal favorites?
I thought this was going to be a thread just about rulebooks so I wasn't going to reply (since I convert everything to AD&D), but I have found a ton of excellent material in the d20 system to plunder.
Necromancer Games -
Almost all their products. They have to be the best made hardcovers and boxed sets produced, but that is, of course, secondary to the quality of their adventures.
- Rappan Athuk Reloaded - This is a monster of a boxed set (and monstrously expensive now I know), but it is one of the best dungeons ever published and if you can get your hands on it grab it, and if you can't, get the three individual modules (but the box set has more material).
- Necropolis - As mentioned in another post, by Gygax
- Tomb of Abythor - As mentioned in another post
- City State of the Invincible Overlord - The classic Judges Guild supplement redone, quite well, as a hardcover
- Bard's Gate - A usefull city supplement
- City of Brass - Boxed set that covers this legendary city on the plane of fire.
- And on and on, I could list their entire line, including their module run with Kenzerco, and that leads to...
Frog God Games -
This is the inheritor company of Necromancer and their Slumbering Tzar mega-adventure is promising to be on the order of Rappan Athuk Reloaded. They have also released as softcover books the missing books from their Kenzerco parternishp line, Demonheart and Eamonvale
Dungeon Crawl Classics -
Over 50 adventures before they jumped to 4e and most of them excellent adventures. From small modules to boxed sets that you could use to kill large rodents.
Pathfinder -
Do they count as d20, I don't care, Paizo is the new TSR golden age company to me. I love their adventure path books, and the indivdual modules and campaign settings as well, and their bells, whistles and cards sets.
Ptolus - what Rappan Athul Reloaded is to dungeons Ptolus is to city supplements. Unfortunately both are ridiculously expensive.
Iron Kingdom - Witchfire Trilogy and the setting books
Wheel of Time - Sadly just a campaign setting and one supplement book that I know of
The Game of Thrones -
Massive setting book. Was anything else published for it?
Orcfest - from Fast Forward games, but haven't found much else to recommend from them
Freeport series - though I think it was Black Sails over Freeport that was best, if it is the book I'm thinking of, huge softcover
There are more, and I'm probably missing some good ones, but this is all I can think of at the moment.