I accidently checked a book to repost that I no longer have in my posession. The book is a wish list book, and it's waiting on pending approval from the wisher. How do  I delete the book when it's already out for a request from another user?

Wait until you get the request, decline the request, and send the requestor a PM to explain.

I would probably PM the wisher and tell them what happened.  I'd ask them to accept the book so that I can cancel the request and get the phantom book out of the system.   If they accept the book, you can cancel, and the wisher will be put back at the front of the line.

You could also send feedback to R&R asking them to pull the book.  Some of us have complained about Wishlist purgatory for some time.  Situations like this are the problem.

Thanks!!