Currently if you open a book from the Catalogs view, the book opens full-screen, but the original window is also still around and visible in the Shell overview:

There's also no obvious way to go back to the Catalogs view, other than closing the window. The Library view has a similar but different problem, because the only way to go back from the book view to the library is via an entry in the primary menu, which is not very easy to find.

I'd propose keeping everything in a single window, and using a standard hierarchical navigation scheme with back buttons (like GNOME Books, along with most other GNOME apps). The main differences would be

• All navigation happens inside the main window
• A "back" button in the headerbar of the reader view
• The book detail view would be inside the main window with a back button, rather than a dialog. The previous/next buttons might have to be dropped in order for that to work.

In addition to being simpler and clearer, this would also have the advantage of working better in an adaptive/mobile context.

Edit: I should note that this is all based on the current behavior in master, if I'm commenting on something WIP that's going to change anyway feel free to close :)