I think you can either decline your friend's shower (you could tell her that your mom offered) or you can suggest to her that you feel uncomfortable dictating how your guests spend their money or putting rules on presents.
That said, if you accept her shower, you really can't dictate the type of shower she throws. If she goes ahead with her "books only rule," you can still register for the items you want and if anyone asks you if you are registered, you can let them know where you have registered. If I received an invite for a book shower, I would still probably look for a registry at the usual suspects (BRU, BBB, Target) and purchase a baby item in addition to the book.
I wouldn't worry about having "too many" books if indeed you end up having a book shower- my DS is 2 and probably has 100 books- we read at least 5-6/day, and he rotates through his favorites, so if you have 10-12 books from your shower, I'm sure you'll end up reading all of them on a regular basis if your LO likes them.