Movie tie-in material isn't exactly known for being high-quality stuff. When it comes to tie-in books, then, there are two avenues that are most often followed:
A) The uncreative novelization that adds nothing to the story and pales in comparison to the film experience. OR
B) The tie-in book, usually for children's movies, that specifically contradicts the movie's plot, characterization, or lesson.
Disney favors the latter.
In the case of Belle: The Mysterious Message, Kitty Richards has written a midquel to Disney's 1991 movie, Beauty and the Beast. As my fellow '90's children may remember, this movie in fact already has a film midquel in the form of 1997's Beauty and the Beast: The Enchanted Christmas (as well the three/four shorts in 1998/2003's Beauty and the Beast: Belle's Magical World.) As for why Disney insists on adding scenes to the original picture instead of moving forward with the characters, I haven't the slightest idea.
But it certainly creates a load of continuity problems. That's the problem with a midquel: if you use the main characters of the source material, you're adding character development that was never present in the original. And that's a big problem in a movie like Beauty and the Beast, where the entire story revolves around the main characters growing closer. If you have important “learning to understand each other” scenes in the spin-off midquels, suddenly the movie itself fundamentally changes.
In the case of Beauty and the Beast: The Enchanted Christmas, this is striking. Tim Curry's character, the organ and former court composer, Forte, is an antagonist to both Belle and the Beast with the goal of keeping them from falling in love. Obviously, this determination to keep them apart and the resulting danger does a lot to bring the couple together... and that makes absolutely no sense in Beauty and the Beastcontinuity. There's simply no room for this character development in the original film.
Belle: The Mysterious Message isn't nearly as jarring, but it's there nonetheless. For half of the book, as a matter of fact, it seemed as if the problem would have been avoided by focusing on Belle and her mission, and ignoring the Beast completely. Unfortunately, the second half dashed that hope; but more on that later.
This short mystery takes place sometime after the Beauty and the Beast library scene and sometime before Belle leaves the castle. In the story, Belle and Chip discover a dusty old book hidden beneath one of the library's bookshelves. Belle starts reading the fairy tale within to her enchanted friends... only for everyone to face disappointment when they realize the last chapter of the book is missing.
As it turns out, the Beast ripped out the last chapter as a child; his tutor then decided to hide the pages, leaving clues for how the young prince could get the pages back. Unfortunately for Belle, the tutor left before the Beast ever found the book under the shelf and the first clue it contained.
So Belle and her friends start to follow the tutor's clues in hopes of finding the end to their story. Unfortunately for the original movie's continuity, they're forced to bring the Beast in on the plan, and Belle and the Beast have several scenes bringing them together as friends, completely in disregard of the movie's relationship dynamics.
Still, for all its discontinuity-making flaws, Belle: The Mysterious Message is a nice, short mystery for any young Disney princess fan. Perhaps a little boring for an adult reader, but a child should find it enjoyable enough.