OK so which site has the most skyrim lore?

I want to know if i can find out small details such as backgrounds to characters, which monster kills the most people a year in skyrim in lore,

so where can i find these

Books can be found in the http://www.imperial-library.info/.

For general questions like NPC background, your best bet is probably the http://www.uesp.net/wiki/Skyrim:Skyrimor maybe the http://elderscrolls.wikia.com/wiki/Skyrim. Those sites have pages devoted to just about every NPC in the game and often they include some background details.

I always forget about that one, Thanks!

Also, as mentioned, UESP has a lot of lore info, particularly here: http://www.uesp.net/wiki/Lore:Main_Page