You wait until you start the lectures to see if it's essential.
More often than not, your lecturer will tell you.
This pretty much. The biggest mistake I made during first year was buying my textbooks straight away - unlike HS, most of your textbooks never get used and they cost a fortune. I probably blew around $800 on textbooks I either never opened or barely used. The best idea is to wait until you have started your classes in order to gauge whether the book is needed. Ultimately, this saved me a stack of money.
Also, for some subjects, the prescribed textbook might not be very useful compared to others. For my Criminal Law class, our textbook was terrible, so I purchased another one which was like a third of the price and ten times better.