thanks everyone who responded. I finally got ahold of the person.

I recieved a book from someone that I didn't order. I pm'd them that I got it and they have never replied. Its been weeks now since I pm'd them. What should I do with the book. Its one that I already have and is wishlisted by other. Should I hold on to it , in case they want it back?

I say if they haven't replied to you in weeks then go ahead and post it and send it on to someone else. 

If I am understanding correctly, you PM'd them once?  If so, I would send one more kindly worded PM stating that you would really like to resolve the situation, but if you don't get a response in one week (two, 3, however long you pick), you're going to go ahead and consider the situation unresolvable, and so sorry for the mix up, and hope to hear from you, and all that stuff included in the PM. 

I agree, Julie.

Was the book from someone who was supposed to send you a different book?  (You can look at your transactions archive, under "My account" to see if there's a cancelled or lost transaction from the same person, and also look at your "en route to me" tab & click on the PM button to see who sent it.)

Hello,

There are many reasons why a person does not log onto Paperbackswap. Not every one has time to log on 3 times per day. However, if you are on a site and might get email back from a person to whom you sent a book, you SHOULD log on at least once a week, or every 2 days. The sender clearly does not have time to follow up on the book they sent, so after 2-3 weeks, and after sending a PM once a week, you can safely, IMO, consider the matter unresolvable. That sounds reasonable to me.

(I know people like this. They think they'll have time to log on but they don't.)

so what happened?