Thanks to Bryan Phelan for alerting us to a potentially useful feature in Apple's Preview application. Bryan was noticing that other applications on his Mac were constantly distracting him from reading his Take Control ebooks until he stumbled on the Slideshow feature in Preview.
With an ebook (or any other PDF) open, just choose View > Slideshow (Command-Shift-F). Your screen immediately goes black and displays just the current ebook page. Click the Fit to Screen button in the transparent slideshow toolbar to expand the page to the full size of your screen. Unfortunately, you cannot see or click on bookmarks, but you can click on any link to follow it, and Web links load invisibly behind the slideshow rather than switching to your Web browser. You can hide the toolbar by clicking anywhere outside of it, but it returns to obscure the bottom of pages as soon as you move the cursor, so keyboard navigation is generally best.
To navigate between pages, press the left and right arrow keys or left and right square bracket keys. Press Escape to exit slideshow mode. The only other keys that do anything are F (for switching into full screen mode) and A (for switching back to actual size). Nearly every other key just beeps when pressed. Bonus points to anyone who can figure out what Tab, R, and L do, since they don't beep.