iPhoto 5 lets the user change and edit fonts for its standard book templates (through the gear-shaped Settings icon on the book toolbar). Unfortunately these settings are all or nothing -- your entire headline is 48pt Futura Bold or it isn't; no mixing of font sizes allowed.
However, OS X's standard type and spelling tools are available in a contextual menu. Just control-click on a selection of text. The standard font pallet is, obviously, under the 'Font' sub-menu. Pick your desired face and size, and just the selected text will be changed.
[robg adds: This trick works on any text in a book, obviously -- so if you have one extra-long caption, for instance, you could shorten it by changing just its font size.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050204164728152