I don’t see any difference one way or the other except for one thing.
Lets say that in the future you wanted to group the authors by genre, Sci-Fi, Historical, Non Fiction, etc. It would be pretty much impossible to do this with authors being a taxonomy. On the other hand as a CPT you could add a genre taxonomy attached to the author as well as the books so that both authors and books could be sorted into these terms.
As far as future proofing it I would go with a CPT.
