The only one I had a chance to read was I, Strahd: Memoirs of a Vampire by P.N. Elrod. It's quite good, and gives a lot of insight into Strahd's character.
I've heard that Vampire of the Mists, Dance of the Dead, Carnival of Fear, Knight of the Black Rose, Shadowborn and Spectre of the Black Rose are good as well.
From what I hear, Death of a Darklord is terrible though.