Tortoiseshell is a much admired decorative material derived from certain species of marine turtle, especially the Hawksbill. It is a thermoplastic material and withstands repeated softening by heating and will harden again when cooled. This made it suitable for decoration using heat embossed, block stamp designs such as the floral design on the cover of 'Les Pseaumes de David'.The photo doesn't supesize, but you can see that the book cover is actually embossed with designs.
Little is known about the use of tortoiseshell as a book binding material... The use of tortoiseshell as a book cover became popular in the 17th and 18th century for religious texts, small bibles, prayer books, and gift books. There was a brief renewed interest in tortoiseshell as novelty bindings for wealthy Victorian book buyers.