Turtleback books are typically the size of a mass market paperback book. The covers are reinforced with cardboard and don't have a dust jacket. Sometimes they are laminated. You often see them in libraries as they hold up pretty well to the abuse of young readers.

susan/vt

What Susan said.  Turtleback are usually paperbacks professionally rebound as hardcovers.  When finished, they look like a hardcover edition of the MMPB...often used by schools and libraries b/c they last longer than just a paperback.

Is that the same or different from "library binding"?

Different, Library Binding usually refers to a hardcover with full color illustrations instead of a dust jacket.  Turtleback usually begin as a mmpb, and are then sent to a specialized binding facility and re-bound (frequently done to classics & childrens library books).  The turtleback books at our school are mmpb classics, a closer look shows the cover is thicker and they are hardcover...otherwise appear exactly like the paperback (in fact, we have some mixed sets of classics with same cover pb and turtleback).

Think of those kids books, the 100-150 page color hardcovers on subjects such as individual States, Countries, Founding Father & Presidential biographies, Dolley Madison, etc.  Those are often published with Library Binding, and did not begin as a mmpb.

It can also include some books that are initially bound in single ply pleather (I think of field identification manuals in specific).