I could really use some support for more metadata such as series. At the moment there is no way to find out the order of your books in a series, you have to look it up on the internet or remember it..
In fact, the whole "library" app could use a revamp. It still feels to me as some sort of unfinished beta product with the functionality stripped to the bones. Where is browsing by series, year, author etc.? (I don't mean sorting) Where is a way to customize the display of books (grid, list, xx per page...)?
And you could've done a lot more. Library should be your main focus on a device like this, not a store.
If you aim for a "simplistic UI" maybe you could put some sort of easy/"hard" mode switch somewhere in there, but please don't restrict your other users because of it. Pleeease....