Perfect bounds books are the perfect bind. Perfect bound books are made up of stacked pages. These are gathered into a book and the edges of the spine are ground off (or notched). When the book block is glued into a paper cover, the glue that attaches the signatures to the spine can flow into the notches or ground-off areas. The increased surface area for the glue allows for more permanent adhesion. The covers and book blocks are then trimmed flush.
Perfect-bound books are square at the spine and use glue to hold the pages, like a paperback book. It gives your books a more professional feel. It can accommodate a page count up to 640 pages (340 leaves 80gsm).
Another big advantage of the perfect bind is that you can also have information printed on the spine.