First of all, Flutie threw a total 91 passes in the NFL over those 12 years. The fact that he stuck around is a testament to what the NFL used to me and not what it is now.
A more modern comparison would be Brees or Wilson, perhaps Kyler Murray. But Book isn't anywhere near as talented as any of those three. Heck Book isn't anywhere near as talented as Chase Daniels for that matter.