Since your schedule should be pretty much set by now (with the exception of the last few people who have not done PittStart) I highly recommend buying books ahead of time. If you are not sure if the book is mandatory or not- email the professor teaching the course and ask. Also email to see if you can buy an older edition of the book instead.
I will say this (for SSOE) every main book in your course list is worth buying (the supplementals are at your own discretion) and are worth the money you spend on them. Textbooks are valuable resources later- so don’t sell them back at the end of the year! I had to rebuy some of my undergrad books in grad school because of how good the background material/text was. Major regret.