your best bet: wait until the professor says they need the book. I've bought books countless times before classes started, only for the professor to say on the first day: "Don't buy the book, we don't use it." Or "The book on the syllabus or website is wrong, get this one instead." Many professors don't require the book the first week- but if you can't get your books in time, the library will always have a copy that you can use (but usually not check out).
This often happens because the department says everyone has to use book X even though book X is terrible. One of the classes I teach sometimes has one of those books. Everyone walks in the first day, and I'm like if you bought the book the bookstore said you needed take it back because it's garbage.