What are the best sources of fluff/background for WHFB?

[Humorous_Conclusion]

This book is a pretty good source of material for the Vampires and definitely covers the different blood lines.

http://www.drivethrurpg.com/product/64520/Warhammer-Fantasy-Roleplay-2nd-Edition-Nights-Dark-Masters?src=hottest_filtered

This is the WFRP 2nd edition Skaven source book. I've not looked at it personally, but I don't see how a decent Skaven book could not cover the clans as they have been an essential part of the Skaven background since they were first created.

http://www.drivethrurpg.com/product/64191/Warhammer-Fantasy-Roleplay-2nd-Edition-Children-of-the-Horned-Rat?cPath=36_4943