This is a sort of sequel to Princess of the Midnight Ball, though I would say that you could read this as a stand alone novel as the only real references to the previous book is a short summary of the story of the Westfalin Princesses, and some of the characters.
As you can probably guess by the name of the story, this one is based on the story of Cinderella (though you won't find the name Cinderella anywhere), with the slight twist that the Godmother is the 'bad guy' (though for all I know she could have been in the original story).
Instead of following all of the Princesses antics, this just follows Poppy (who is 16 and the twin of Daisy, I believe she is 4th oldest of the 12 Princesses). Poppy is a heroine full of life, she's mischevious and a bit devil may care, and this makes her story and journey even more enjoyable as you see how her feelings and emotions progress. Throughout the book you also see her coming to terms with, and dealing with, the events in the previous book, and becoming a more independent and happy girl.
I actually found that I was surprised at the ending to the book, like I said in my post earlier today about Princess of the Midnight Ball, it was predictable in the sense that it was a happy ending where good vanquished evil, but the love story that took centre stage was not the one I expected to.
I believe that in this novel, the author has taken a step up from Princess of the Midnight Ball, the story was more fleshed out and there was definitely more depth to them, though this could be something to do with there being far fewer important characters.
I would recommend this book without a shadow of a doubt, though I would also say that you should read Princess of the Midnight Ball first, just because it will give you more of a feel for Jessica Day George's writing style, and indeed for Poppy, though as I already mentioned, reading it before Princess of Glass certainly isn't necessary!