This binding style sits somewhere between a paperback and a hardback binding and the end result is a lightweight book with a flexible cover, usually with a round spine and endpapers. The book will lie fairly flat when open which makes it convenient to use.ith either version, the resulting book is lightweight, great to use and has a high quality feel. One interesting add on that flexi binding offers over cased, is that you can add flaps to the cover-as you might on a paperback. It is of course, also perfectly possible to add a jacket to a flexi bound book.