I believe Oliver Bowden is a pseudonym, he's the fictional character that's in charge of Abstergo Entertainment in Black Flag. But from what I can tell the books are all still written by the same author, Anton Gill. I know there's adaptions of Brotherhood, Revelations and Black Flag but I believe there's an original story for Altiar, Ezio and Connor as well. Edit: Forsaken, Renaissance and The Secret Crusade. They may borrow some from their respective games it looks like.