This one's called the Chain Stitch, not to be confused with the Chain Stitch that you use to
This is what the open book, between two signatures, looks like:
Were you to open the book within a signature it would look the same as any old book, with the stitching running vertically down the middle. This book is filled with blank pages, as well as glassine sheets (thin transparent paper for archival purposes) and found pages from another old book, which you can see above. The spine has a nice loose but strong feel to it, and is very flexible (obviously).