"The Vampire Companion" is a very exhaustive encyclopedia of most of the terms, characters, locations, and events in the Vampire Chronicles series. It only covers the first five books in this series, though ("Interview With the Vampire", "The Vampire Lestat", "The Queen of the Damned", "The Tale of the Body Thief", and "Memnoch the Devil"), which may bother more die hard Vampire fans. Information on the other four books ("The Vampire Armand", "Merrick", "Blood and Gold", and "Blackwood Farm") is obviously missing since they were published after this book.
Just about everything's included in this mammoth 500+-paged book: maps, illustrations and photos, bits of Anne Rice information, a time line, in-depth character biographies, etc. Even the unpublished short story of "Interview With the Vampire" is included, which originated the novel one year later.
If you're an Anne Rice fan--and especially if you enjoyed the Vampire Chronicles--then I highly recommend this official reference guide to her popular vampire series, even though it only covers half the books in the series. (Hopefully, there will be another revised edition published soon.) But for what it does cover, "The Vampire Companion" is very thorough and worthwhile to Vampire fans. This is certainly a book worth owning, since it answers just about every question you've ever had about this series and then some.
Also recommended: Katherine Ramsland's "The Witches' Companion."