Teams aren't going to spend a draft pick on someone who would essentially be relegated to being a backup QB at best, given his size and arm strength. Maybe in the older days of the 12 round draft, he could have been a late round pick, but with today's draft being only 7 round, it's simply not going to happen.
Think of it this way... Someone like Ty Detmer (similar traits, although Book has a clear advantage on mobility), who was a QB god in the world of college football, was lucky to be drafted in the 9th round, back in 1992. The knock on Detmer was that he would have been the perfect franchise QB, if he were 3 inches taller, 30 pounds heavier, and could throw for more than 40 yards.
That being said, I'm confident that someone would offer Book a priority free agent contract, and he would certainly have a legit chance of sticking around.