Jeez, you guys, welcome to the real world. Especially if you're going in for business, you should relish this little exercise and milk it for all it's worth, because it is what will separate the men from the boys once you start trying to succeed.
The prof wrote the book? And it sucks? Okay, the lesson here is not "how do I convince the prof that I know more than he does, the moron" but "how can I use this to my advantage?". Answer one is MEMORIZE that damn book. Toss references into casual classroom conversation like "as it says on page 112, the blah blah blah" or whatever. Step two is to research the market. Is his textbook the top seller in all the land, or is it used solely in his classroom? Find sales statistics, find reviews. If it's a widely used book, suck up, er, congratulate your prof on being a leader in his field. If it's a dud, go heavy on the "visionary" or "clever thinker" or "hey dude, you actually got your RA's to write a book", whatever you think the prof will respond to.
Step three, read a bunch of other texts so that you can say "in your text apples are purple, but in Smith & Jones apples are blue. Could you tell us why Smith & Jones are such idiots?" This is powerful stuff. Anyone who can succeed at this in the real world will be king of the hill.
Good Luck.