You don't know what The Watcher's Guides are? The three books? Tried googling...? Hehe. They're everywhere. They are essentially just episode guides for BtVS but with inventive extras and lots of cast and crew interviews which are really cool.
The Watcher's Guide Vol 1 covers Slayer and Watcher lore, demon lore, vampires, character profiles, cast and crew interviews and info, and seasons one to two. Also the music of the seasons
The Watcher's Guide Vol 2 covers more BtVS lore, more character profiles, more cast and crew interviews, photos and episode guides for seasons three and four.
The Watcher's Guide Vol 3 includes the same type of material but details seasons 5 to 7.