I've never seen this before. I purchased a book today, but discovered when I got home that I already had a copy. [Can't return it since it was part of a clearance/store closing sale.] So I figured that I'd post it here, since I knew it was wishlisted. I typed in the ISBN, and it didn't come up as in the system. Curious, I looked at the inside of the book and found a DIFFERENT ISBN on the publisher's information page. This ISBN matched the Hardcover version of the book, while this version is a paperback.

Obviously, I can't post it under either version right now, but I'm curious as to why the Hardcover ISBN was in there but not this one. I've seen multiple ISBNs inside before (rarely), but one always matched the ISBN on the cover. Has anyone else ever come across this before?

[If you're curious, the book is Little Brother by Cory Doctorow, and it's a fantastic read! I read it from the library and decided I wanted a keeper but apparently already did so!]

I have seen this before. I believe that if the book is a paperback, but the ISBN on the inside is for the hardcover version, then the one on the back cover is the correct one. It may just be that the book is new and hasn't been picked up by the system yet. I have heard that just searching for a book here prompts the system to add it. So, maybe it'll show up soon.

That's definitely possible - the version I have is also paperback but a different size. The one I got today is closer to Mass Market size, but it's wider.

With modern PB's there is another ISBN# along with the UPC on the inside front cover.  Is this a third one, or does it go with one of the others? 

Double-check, too, to be sure your copy isn't an Advanced Reader Copy.  Here's some info on ARCs:

An ARC  (Advance Reader Copy)  is an advance release of a book, usually sent to reviewers before the final copy of the book is published.

• Advance Reader Copies are also known as Review Copies, or Uncorrected Proofs.
• They are softcover books and often carry the same ISBN as the hardcover first-edition that is released later.
• On the cover, they will be marked 'Advance Reader Copy' or 'Uncorrected Proof' or 'Review Copy' and will usually also say 'Not for Sale'.

ARCs cannot be Posted to a PBS Bookshelf for swapping, since the ARC cannot be depended upon to contain the same story/text as the published book.

I had a weird one once, it wasnt an ARC or anything but it had 4 different ISBN but didnt say after it like paperbk etc. I had to go through another books and check the date and the pages etc to finally figure out which one it was.