[GeoffR]

How are you entering the library? If you enter using the menu at the bottom of the home screen then whatever sort order you previously set should be used. But if you enter via a Library tile on the homescreen then it always sorts by most recently read.

Edit: To add a bit more of an explanation, the Library tile shows up on the homescreen when you add new books to your library, so the idea is that when you tap it you will see the new books you just added.