Nice little tale. A few things jump out to me though. One is that the evil magician randomly selects someone off the street and expects him to find the Book of Smith for him. It would be a bit more believable if the hero had some connection to the book beforehand, or perhaps to the rat caretaker. The rat trusting the book to a random person is a little unbelievable too. That could be fixed with perhaps a "truth spell" or something or giving the two a connection as mentioned above. Also, it seems to me that the magician turned into a normal, yet still quite large and quite well tooth-and-clawed tiger would still be quite capable of putting up a fight, rather than turning and fleeing. (Though it does make sense he doesn't want to get zapped again.)
Still, enjoyed it, and fun anthro to normal-form transformations here. Looking forward to reading more of your work.