I have a paperback version of The Alice Behind Wonderland by Simon Winchester that I've had to post without an ISBN.  As far as I can tell, it's the same book listed in the system as a hardback, but when I put the ISBN on my copy in the system, it comes back with no results.  There are 19 members currently wishing for the hardback version, but there is no paperback version listed.  As far as I can tell, this may be a Book of the Month Club edition as that is the only return I get when searching the ISBN.