On July 17th I requested a book. The book was supposedly mailed on July 19th. Weeks went by, no book. On the day the book would have been marked lost, I received the book (August 16th). The postmark was August 12th, but the weirdest thing was it was mailed priority mail (4.60) for a very small book.

I am wondering if the book was possibly sent back to the sender for some reason?  Has anyone had anything like this happen?

I'm not complaining. I was actually quite surprised to actually receive a book I thought had gone missing in the mail. I was just slightly confused!

Only the sender can tell you for sure.  Usually when I receive a book like that, the sender forgot to send it, and just sent it priority or overnight (yes, believe it or not!) to make sure it arrived on time.

I've had a book come back to me once or twice for address problems or an incorrect zip code, but whenever that happens, I always PM the recipient to let them know the book will be delayed.

My guess is they didn't send it when they should have.  I had that happen one time.  The sender actually PM'd me to let me know that she didn't mail it when she should have and to make up for it was sending Priority Mail.