I ordered the paperback version and got the hardcover - different ISBN - which I'm not too happy about since I really don't like hardcovers and I'd intended to keep the book.  Do I mark it received?  I still want the paperback, so do I have to go back to the end of the line?  I looked through the help docs and I'm still not really sure...

I'm also uncertain as to whether I ought to ask for a credit refund, send the book back, etc.  The hardcover version also has people wishing for it.

Mark it RWAP and you will be returned to the front of the line. 

You mark it received With A Problem - Wrong Book. It is up to you if you want to ask for you credit back, I usually do on wrong bindings because I will be requesting the correct version and it will cost me that credit. It is also one of those posting areas that I want the sender to understand they cannot be doing that. Some just post to the wrong binding because it has a Wish List.

If they want the book back, they should be giving you an extra credit or sending you the money to cover the cost.