I'm sure others will discover this soon enough by accident as I did, but I haven't seen in mentioned anywhere else yet...
If you double-click a photo on a book page, a slider will appear above the photo, allowing you to scale the photo. Once scaled up beyond its default size, you can click and drag to move the photo around in its frame. Finally, double-clicking a second time launches the edit mode.
[robg adds: As far as I can tell, the 'scale via double-click' trick isn't documented in either Help or the keyboard shortcuts pages in iPhoto 5, nor is it available from the menubar or the contextual menus (isn't that a no-no in Apple's HI design guidelines?). It's also a very useful feature, as you can control exactly which portion of your image will print, and you get a caution flag if you've zoomed in too far for the available resolution.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050202053000417