I’m not sure why, but I’m often a bit leery of books written by two people. People often collaborate on things like screenplays, teleplays, songs, and probably a bunch of other creative endeavors. But books… I think it’s because most of the books I know that are written by a team are pulpy crap. The exception being something where a great person wants a story told and has the foresight to get someone else to write it. Even then though, it’s often less than good.