This information is important to a bookseller because when we understand the mechanics of a book, we are better able to judge whether it was:
(a) well made and
(b) has stood the test of time.
Books begin their lives as groupings of consecutive pages called signatures, which have been printed, trimmed and folded. Signatures are organized in order, have the covers added and then are bound. There are several types of binding: sewing (the most traditional) using thread, stapling, and perfect binding (using glues).
To tell whether your book has a sewn binding--a generally desirable trait as it makes the book more valuable--open the book, turn it on its side and look at the headband (guide to parts of a book). According to Leonard's Books you:
can (sometimes, or often, or usually) tell if the pages appear to be side-by-side in neat little folded groupings like tiny booklets. These booklets are called signatures. This is usually a clue that the binding is sewn.
But not always!
Now, if you can get to the center of one of the little signatures, look for sewing or stitching running down the length of the spine edges of the pages — in the gutter, in the center of the folded middle page. If you see threads, you have a sewn binding.If your book is beginning to come apart, you can look in between the signatures and see the threads; note that a mesh-like cloth along the spine is not threads, but a material used in glued bindings.