How to pick a book to alter

I'm starting a new AB and, since we were having a discussion on ABC on the best book to alter, I thought I would show how to choose a book whose signatures have been sewn.

Check the spine. If it pulls away from the signatures and has threads showing, it is sewn. See the gap? If you don't see a gap, use pages from the book, but not the book itself. If there is a gap, but you can't see the threads, use pages from the book, but not the book itself.
If you click on this photo which will give you a bigger image, you will see bits of the thread that separates one signature from another. At the top, you see the first page of one signature. At the bottom is the last page of another signature. As an aside, I really CAN spell. I just didn't notice the spelling mistake until I clicked on a large image. Was I surprised when I saw "fuirst." Oh well, I'm sure Photoshop has spell check, but I don't know where it is.You can see the the thread in the center of a single signature.
This is a single signature in this book. A signature is a set of pages bound into the book. Signatures usually have an equal number of pages.

Hopefully this will help at least a few new ABers, and prevent a heartache when their partially completed book falls apart (and I assure you, it will).