Book is much closer to Clements that Theismann as a quarterback. In Theismann you are talking about a guy who played 11 years in the NFL and is a former NFL Most Valuable Player, and arguably one of the 3 or 4 best quarterbacks to have ever played at ND. While no Theisman, Clement was a good enough passer to be a 7 times CFL all-star, a CFL mvp winner, voted one of the 50 greatest players in CFL history and a member of the CFL Hall of Fame. He also played one year with the Kansas City Chiefs as a backup quarterback. Again, you are vastly underrating Clements as a passer.
Book is also closer to Clements as a runner. Both could/can scramble and keep plays alive and were also good runners when they ran/run the ball. However Theismann was a much better and dangerous runner. Theismann even returned punts a few times when he was with the Redskins. I don't know many NFL quarterbacks and can claim that distinction.
So I stand by my position that Book is much closer to Clements as a player than he is to Theismann. But that is not a bad thing, as Clements is one hell of a player to be compared to.