Maybe, a book previously owned by a library that they no longer wanted...
Or maybe it's that kind of thing where the library binds a bunch of the same things, let's say magazine issues, together so they can be catalogued or somethin'.
I emailed the people from some library binding dork site and they said this:
"Library Binding is hard cover with a Buckram cover (Library grade vinyl), more or less an Industrial grade binding made for several uses. Not fine leather or cloth, which is more delicate."
A library binding is sort like a special edition book. The inside cover is in a differant end sheet, usually some kind of a neat pattern of some sort.
I used to work in a factory that made books, and have gotten alot of Stephan King books in both Library and regular bindings. The book is exactly the same except fro the end sheet.