Codicies both past and present are the best bet, as well as rulebooks. Understand that as far as fluff is concerned it rates:
- Present Codecies and Rulebooks
- Previous Codecies and Rulebooks
- White Dwarf (if you can even find fluff there these days)
- Forgeworld publications (IA, Xenos, etc)
- Lexicanum and Black Library
I rate Lex and BL on the same, and the bottom, because Lex pulls from all of the above sources and Black Library. Black Library is dubious, because there are some terrible writers in there who have done some fairly stupid things (Space Marines with Shuriken Cannons and MultiLasers). Typically, Black Library is only "official" if you can't find information pertaining to the question anywhere else (Soul Drinkers, for example, cannot be found in GW publications).