*Important disclaimer: Do not take this as legal advice. This is just my impression of the legal issues based on my own research and experience. I am not a lawyer. I am not responsible for any issues you run into by following what's said here or by using the scrapers.*
Class-book data is a set of facts, similar to a phonebook. Therefore the bookstores can't claim copyright on a creative work. However, scrapers have been prosecuted for scraping facts on the basis of "tresspass by chatell" law.
For the following reasons, it is likely that scraping of bookstore data will not/cannot be successfully prosecuted under "tresspass by chatell" law:
When I first started TextYard I was very afraid of breaking the law. In retrospect, I should have taken the risk right away. That is my two cents. Contact me if you want to discuss the legal issues in more depth (firstname.lastname@example.org), but know that I am no expert.