www.wowwiki.com is a good place to browse, but I hear it's not always reliable since everyone can write in a Wiki and mistakes aren't always caught. Still, some good pages to start with are:
www.lunarfalls.com/WarCraft/index.html : another huge collection of in-game books, conversation transcripts, and lore excerpts from the Warcraft game manuals
It still seems to be heavily under development and some of the things there make me blink (druids only hibernating "periodically" flies directly in the face of the events in WC3). Still, here are some interesting articles: