I feel really bad. (But some books have the same title!) I sent a message to the sender, and I know it might have already been sent. (The wrapper was printed) Anything else I should do?

Are you the sender or the receiver?  It sounds like you are the receiver, and if this is the case, you've done all you can do.  If the book has already been mailed, the best thing to do would be to mail it back (at your expense) and ask the sender for your credit back.

Oh ok. I didn't know if I could do that. I was thinking since it was my mistake I'd just put it on my shelf of books to swap. I'm the receiver.

I'd think you'd just have to repost the book to your shelf. I can't see how the sender has any obligation to return a credit or take a book back just because the "buyer" made an error in requesting it. I'd not be inclined to do it. I am credit poor and would rather use a credit earned to buy something new for myself, not rebuy a book a just sent out.

Well either way, lesson learned. Thanks Lisa and Melanie.

I just did that too.I had WL both hardback and paperback ,I wanted it any way I could get it.then realized I had ordered both.D'OH

Fortunately, the sender had not mailed it and was kind enough to cancel.Whew, crisis averted.Makes me be more careful and pay attention.