The Bob Dylan Encyclopedia by Michael Gray.
I saw it in the library and picked it up, not knowing what to expect, but as a Dylan fan I figured it would be at least a little interesting. It's actually brilliant -- very well-written, informative and humorous. The author doesn't just cover Dylan exhaustively, but Dylan's influences (old-timey blues and folk musicians), the poets Dylan "borrowed" from, Dylan's contemporaries, all the main and bit players in Dylan's life and career, etc.
This book is to Dylan (and, in a way, American music) what Bill James' Historical Abstract is to baseball. You can open it up to any page and find something fascinating to read.