Shemeska
Adventurer
As tempting as it is for me to just say 'any monster book, monster ecology book, and anything with a drop of planar material in it', I'll stick with, in general, anything that is primarily fluff and flavor text rather than being chock full of crunch and rules.
My reason? Come any future edition, I want books that will still be useful under altered rules, rather than being completely and wholly useless under a changed system of game mechanics.
				
			My reason? Come any future edition, I want books that will still be useful under altered rules, rather than being completely and wholly useless under a changed system of game mechanics.