Provide a Reader mode similar to Safari
Safari's distraction free "Reader" mode graying out the background and just providing a view of the content is very nice. Bookle could potentially do something similar (without switching to full screen mode, and preferably via a keyboard shortcut) where the right side display of the book content was focused for reading with everything else on the monitor grayed out in the background. Ideally that would work one monitor in a dual monitor configuration leaving the other monitor operating normally, or even optionally having the 2nd monitor grayed out for "fully" focused reading.
Interesting idea - I think Bookle would probably have to simulate a full screen mode to be able to write over the area outside its main window. We'll keep it in mind!