I agree with Bassmaster. Media mail for common books, Priority Mail for higher dollar books. Media Mail can be a bit slow and parcels tend to receive some abuse. I'd recomend you look at Priority Mail Flat Rate boxes if that is the way you decide to ship. They are free at the post office and you can also order them online and the post office will deliver to your address for no charge. Free Priority Mail tape is also available. You have to order that by phone. Your postmaster can furnish that number. The flat rate will save you, and or the buyer some money on heavy items. Priority Mail seems not to get too beat up and tracking and the first $50.00 of insurance is, "free".
Good luck with your books!