What do you do if you receive a book that is not the one requested?

The Help Section I received a book I didn't request! will explain a lot for this situation. What it boils down to is -- did you receive the wrong version of the book you requested (ie the content is the correct story, but the "packaging" is not, like a MMP instead of a HC or an ARC)? Then you would mark the book RWAP- wrong book and can ask for your credit back if you want it in that process or -- did you get a completely wrong book? Then you would not mark the book received and should contact the member to see what happened. If you eventually get the correct book, then you would mark it received otherwise you just let it go lost in the system.

The sender (not the receiver) has the option to ask PBS to cancel the order if it wasn't a case of switching books when wrapping.   You would have to wait until the book is declared lost if they don't ask for it to be canceled.

And if it was switched books, the sender will know both receivers.  It is then up to the sender to decide how to fix the books they switched.  You should get a credit or postage from the sender if they ask you to mail the book back or to the other member.