Always pack your book in some kind of plastic. This is to protect the books from the elements. Not all books are delivered on sunny days, and not all fit into an individual’s mailbox.
Sandwich the book in between two sturdy pieces of cardboard just slightly bigger than the book. This prevents bent corners.
A cardboard box is probably going to be the safest way to package a book. If you’re on a budget, you can get by using a padded envelope if you use the above tips.
There are no rules set in stone for the packaging of books. The main thing is to put in a little extra effort and make sure the book is water proofed and able to withstand a little abuse.