I had a wishlist book come up and ordered the book.  I received the package in the mail but it is the wrong book - wrong title, wrong author, wrong ISBN.  I PM'd the sender but haven't heard anything back yet.  The book I did receive isn't one I would have ever ordered and I certainly do want the wishlist book that I did order.  Now what?

http://www.paperbackswap.com/help/help_item.php?id=19

Here is the link to the help doc.

Basically you need to contact the sender and see what's up.

Don't mark the book Recieved.

Just wait for the sender to get back to you.  The most likely reason is that the sender switched labels.  This has happened to me twice a  receiver and  once(this past week) as a sender.  The sender can send both people with wrong books a credit for postage, and they can forward the books on to the correct parties. 

I know it's frustrating, but mistakes like this happen sometimes. I did it when I mailed 20 books out (in 9 orders) and I switched labels on 2 orders with the same number of books.  I felt really stupid when I heard about it.