I have posted a book which is wish listed but has since become unpostable (big spill!). Is there any way to pull it off my bookshelf? I sent a PM to the person who it is offered to (they haven't responded yet), but if they decline the book has to go through 9 more people on the wish list. Any way to avoid this?

don't ask them to decline!  They'll lose their place in the WL line.  They need to accept and then you need to cancel the request.  This way they go back on the top spot.  It won't let you cancel until it's been accepted.  So pm them back and tell them to accept the book so you can cancel the request.  Hopefully you won't have to wait for several people to let it time out before someone will accept it for you.

This happened to me once and I had to go through a few people.  When it was finally accepted, I PM'd the person and offered them another book off of my shelf because the book was unpostable and shouldn't have been posted (my error).  I also told them that if they didn't want the book I would cancel it so they didn't lose their place in line.

it worked out well in my case but the main thing is wait until someone accepts and go from there

Mary and Leslie are right, don't let the requestor cancel because they will indeed go back to the end of the Wish List line.  Once you cancel, the book is automatically removed from your bookshelf, which is what you want to do anyway. You can actually just go ahead and cancel the request now.  Keep a note of the requestors PM address if you want to continue discussions with her.  Just remember that it is against PBS rules to send a damaged book for a credit.