I was browsing books and I notice that some books are described turtlebacks rather than paperback or hard cover. What is a turtleback book?

According to a Google search it's a library bound book at a trade paperback price.

I've got some turtlebacks, they're the size of a regular paperback, but with a hardcover that has the picture from the paperback for it's cover.  Libraries use them because of the increased durability.  Sometimes books like this are instead called permabound.  They're basically glorified, rebound paperbacks, but I think they're awesome.

That's the term I had heard Jimi..."professionally" rebound for greater durability.

They are usually ex-library books. I haven't been that impressed with them, but what I have found is that quality varies WIDELY depending on what company rebound it. 

It's usually a paperback that has been rebound with solid, hard covers. Once rebound with hard covers, they're used in classrooms and last longer than paperbacks. "Permabound" is one company that does it. The book is usually marked on the back of the book by the company that rebound the book.

I thought it was a shirt with a high foldover collar - oh wait, that's a turtleNECK!