I have done a book shaped cake before by putting two 9x13 cakes together and it worked very well. But my question is, I have an order and it's only for about 15-20 people. So one 9x13 would be big enough BUT is it big enough to make a book??? Will it look right?? I'm assuming I would shape it so the pages would be 9" long and each page would only be about 6½" wide. Has anyone done this out of a 9x13 pan before?

Yes... :0)
It's just a matter of making it smaller like you envision. I just took 1 9x13, cut it in half, trimmed it slightly & built the book. This should give you about 18 servings.

Have fun!