I work in a library (and one that buys RPG books for check out), so luckily, I'm surrounded by stuff to use for fluff!! I think my favourites are Hamunaptra (That is WAY misspelled, it's an egyptian boxed set) from Green Ronin, along with Black Company also from them. I also love GURPS books for fluff. Also, maps and guide books from AAA are great for fluff mining, and I like mining myths from other cultures as well.
Worst....gotta be Midnight.
Worst....gotta be Midnight.