I've been reading through The Black Dagger Brotherhood An Insider's Guide over the past several days. The short-story about Bella and Z is wonderful. Zsadist has always been my favorite of the brothers and I loved to get a continuation story for Bella and Z.
There are also interviews with each of the brothers, an interview with the Warden and the brothers, deleted scenes, and so much more. I really enjoyed reading how each book came about and loved reading more about each of the brothers. The additional bonus of seeing a sneak preview of Lover Avenged, which comes out in May 2009 only makes me want May to get her that much faster.