I've been in lurk mode for a while, but I recently came across a program that is relevant to this discussion. With the typical desktop/laptop screen, the aspect ratio is too wide for fullscreen reading, unless your reader supports dual page mode (FBReader and others do not). This situation is worse with modern widescreen displays.
One solution to this problem is a program called Ghoster, which can be found at the URL below (scroll almost to the bottom). What Ghoster does is to dim or even black out the screen, except for the active window. So, I just size the FBReader window to suit my tastes, run Ghoster and reselect FBReader as the active window.
Note that to deactivate Ghoster, you right-click the icon in the System Tray. I also changed the default transparency level to make it easier to see the other program windows that are dimmed. Around 200 works for me. You can also try different background colors. I just stick with black (000000).
Ghoster:
http://www.donationcoder.com/Software/Skrommel/