While working on our holiday card this year, I became annoyed by the fact that the body text in the Year In Review theme was centered, making it ugly and hard to read. I spent a few minutes searching and found an iPhoto Hot Tips page from Apple that gave me the answer.
If you format text in TextEdit (or presumably many other word processors), you can paste that fully formatted text into the card's text field in iPhoto and have the card retain all your original formatting. This is true even of fancier formatting options like line spacing and text alignment, and you can also set and use tab stops. So for my card, I simply copied the text out of iPhoto, pasted it into TextEdit, changed it to left alignment, copied it again, and pasted back into iPhoto.
The same trick works for text in iPhoto books, so there's no need to be frustrated by text formatting options there either.
The iPhoto Hot Tips told me something else I didn't know, which is that you can Option-choose a background color from the Background pop-up menu to set the color of the inside of a card to be different from the outside. I don't see myself needing this often, but it could come in handy.
The rest of the tips on that page, save one, are pretty much obvious from looking at iPhoto's interface. The one remaining non-obvious tip is that Apple recommends using the Sharpness slider in the Adjust panel as the last thing you do, in order to end up with the best image quality. No explanation is given for why this is the case; my only caveat is that you want to use the Reduce Noise slider as nearly the last thing because it's so CPU-intensive that further edits can become sluggish.