What are your top 5 RPG sourcebooks based solely on the quality of the fluff/lore they contain?
Hard mode: No GURPS historical sourcebooks.
Life on Terra Nova, 2nd Ed. The perfect introduction book for Heavy Gear.
Tribe 8 doesn't get enough love on here. Despite all the metaplot bullshit, it had some excellent writing.
I could write out the Delta Green supplement books but you get the idea. I think Delta Green is probably one of the best sourcebooks for Call of Cthulhu, if not all RPGs, ever made.
DnD 3.5e book detailing world-threatening villains and their organizations. Exceptionally well presented, it includes scenarios for encountering and learning more about each villain at low, medium, and finally high levels. Gorgeous character illustrations and good map layouts for lairs.
>Paranoia XP Troubleshooter's Manual
I have never laughed so hard while reading a sourcebook. Messages from a mad computer are written into the borders of the pages and it comes with little team badges you can hand out to the players illustrating their jobs, responsibilities, and likely ways that they'll die in the line of duty.
>Rappan Athuk: the Dunegon of Graves
A deathtrap dungeon by the excellent writers of Necromancer Games, focusing more on puzzle elements than direct encounters. Bring backup character sheets.
Covers a broad array of topics from the usual odd treasures and encounters to extraordinary campsites, strange smells in dungeons, and "short encounters for short attention spans."
>The World's Largest Dungeon
The megadungeon of all megadungeons. It is subdivided into multiple smaller dungeons (those each of these sections is a module's worth of material unto itself). The quality between each dungeon's sector varies from the truly awesome (a volcanic mountain in the middle of a lake of lava houses a gigantic demon at the heart of the dungeon) to the less-so (a sector basically nothing but generic undead encounters in a huge crypt). The real appeal of the book is how different sector and factions in the dungeon interact with each other either via trade, alliance, or war.