"The Witches' Companion" is a very exhaustive encyclopedia of all the terms, characters, locations, and events in the Lives of the Mayfair Witches trilogy (The Witching Hour, Lasher, and Taltos). Everything's included in this mammoth 500+-paged book: maps, illustrations and photos, a Mayfair family tree, bits of Anne Rice information, a chronology of important dates, in-depth character biographies, European and American history--the list goes on.
If you're an Anne Rice fan--and especially if you enjoyed the Mayfair trilogy--, then I highly recommend this official reference guide to her popular witch series. It's definitely worth owning, since it answers every question you've ever had about this trilogy and then some.
Also recommended: Katherine Ramsland's "The Vampire Companion".