Have you updated to version 1.1 or later? In the current version we added a fullscreen guide on how to open the menu and returning to the library when you open a book for the first time. There are also quite a few bug-fixes and improvements that really enhances the user experience so we really suggest you update if you have not done so already.
In any case, what you do is this when you're reading a book. Swipe the left edge of the screen (to the right) to slide open the menu. Here you can tap on "My library" to return to your library.