This pretty little book is an example of a longtsitch binding (not to be confused with the historic longstitch, as seen in books 1 and 2). The structure is often accompanied by a paper wrapper, which fits snuggly around the bound book to protect it from daily wear and tear. These covers are more sophisticated versions of the ones made from brown paper bags to cover your math book in middle school. I made this version sans wrapper because that Cave paper cover material is just too pretty to hide. The key to this structure is to cut slits along the spine of the cover that are wide enough to allow for the thread to pass from the inside to the outside of the book (note the width in the detail photo). If you simply cut a slit, it is too narrow a space for the thread and puckering will occur. That said, avoid cutting slits that are too wide, as it creates a sloppy binding and one that will then beg for a protective (and masking) wrapper.