I have a feeling it's in this list somewhere if you want to peruse it and see if any titles pop out at you. This list http://www.scifi.darkroastedblend.co...-novel_06.html
or the post-apoc "novella" list.
off topic note: I noticed thanks to a mobilereader, that first place one is here on Mobileread.com
note 2: I don't necessarily agree with any of their rankings but you don't see too many post-apoc ranked lists so thought I'd share it.