Teen fantasy novel that can be read forwards and backwards for different perspectives on the story

[Danya]

I don't remember much of the plot of this book, sadly, but I think I read it at a library in Denver, CO sometime between 2007 and 2011. It was a seemingly old book even then, approx. 200 pages, hardbound, with a fairly small surface area (would fit with my stack of "small" books). The cover was black or navy blue and had pink and blue letters for the title. The most fascinating part of the book was that you could only read the front side of each page as you read it from front to back because the back side of each page was written upside down. Then, when you finish reading the book the first time, you can flip it over vertically and read it from the other main character's perspective (technically reading it back to front). I think the main characters were a boy and a girl, set in castle times. Can't remember if they were siblings or each others' love interest. Is this enough info for someone to help me find it? I am hoping the unique orientation of the book would make it stand out.

[Becca]

Here's a list of Double Sided Books, in case it's on there.