The Frog Princess, by E. D. Baker has (it seems only appropriate to say) spawned a full series of prequels and followups, and this is the only one I've read. It seemed a good place to start. Also it's on sale for Kindle right now for only $2.49.
I liked Baker's Wide Awake Princess, so I had wanted to give this one a try and the deal was too much to pass up. It's another quick read, aimed at Middle Grade readers so zippy for me to get through. And rather enjoyable.
Although anyone hearing the premise of the story might get it confused with a similarly-themed Disney feature, the stories are different enough. Yes, the Princess does attempt to kiss a frog and turn him back into a prince, only to have the spell backfire and turn her into a frog as well, and yes, there is a swamp involved, but beyond that the characters and situations are their own.
I really like the magic of this world. Not too complex, but well reasoned and definitely the kind of magical detail that makes for good escapism. My favorite character was actually the witch, Princess Emerelda's Aunt Grassina, rather than the heroine herself, but she was also clearly a good role model for the future queen.
Overall: a quick, enjoyable read. I think I shall look for the sequel at some point, although the story resolved such that I don't have a desperate need to know what happens next.