I agree with this...and somewhat with Irish#1's idea that Book is a very good game manager.
Book has been great. But his INT's and some other miscues have demonstrated that he's not always reading defenses, but rather, trying to simply go with a pre-snap notion. I think as teams change and disguise coverages, he'll have some struggles. I think a large portion of his completion percentage has been based on his ability to execute the Long/Kelly offense.
Alex Smith is a guy in the pros who has won a lot of games, been mostly accurate, and has athletic/running/scrambling ability. He's also considered a game manager. I see Book in this mold (granted, we still have a relatively small sample size to go off of). I think he can execute the hell out of an offense, is accurate, and has pretty good running/scrambling abilities. His ability to look downfield during/after scrambling is great too. If coverages confuse him and/or the gameplan isn't great, I'm not sold (yet) that he can go out and win games though. Thus, I think a title of game manager is probably appropriate. It's not a knock on Book. Can he continue to grow and evolve? I don't know. I don't think any of us do.
A lot of QB's from OU, Texas Tech, Washington State, and other spread passing teams have put up huge numbers and been very accurate, but have also not been all-world QB's- they've just executed the offense well. They've been good game managers.