The cover is then glued to the outside edge of the signatures. A designer should take this into consideration and should allow a bit of an extra gutter for this. You'd be surprised at how often I see this goof. This is the method that one sees most often on hardcover books. Signatures are sewn together, glued to a gauze strip and then glued to end papers which are attached to the hard covers.
Comb binding machines are pretty inexpensive and the spines can be removed and reattached as needed.
My Design Studio.