The two princesses of the title are beloved sisters, Addie and Meryl. However, their personalities couldn't be more different. Meryl longs for adventure, but Addie is shy and timid. But when Meryl is struck by the dreaded Gray Death, Addie is motivated by necessity to strike out on her own and seek the source of and cure for the disease that is epidemic in her land. Alone, she must face a dragon and learn that she too can be brave.
This was an entertaining enough light fantasy, but considering that the author is a Newbery-Honor winner, I expected more from it. It was a little too "message"-y as far as the "coming of age" theme, and I didn't really enjoy the somewhat silly personality of the dragon. I also felt it really neglected the stated situation that the Gray Death was a serious epidemic in the country, but all the way through, Addie really cares only about her sister, not about her people who are suffering equally - and that's never really addressed.