You may have seen a headline on Facebook (it's trending, whatever that means) about a fan theory that supposes Belle's favorite book is actually about Aladdin. It's a swell idea based on a 2013 post by Tumblr user PetiteTiaras interpreting a line the song "Little Town" in Beauty and the Beast, but it just doesn't hold up.
For starters, Aladdin isn't a "prince in disguise;" he's quite the opposite.
Also, a little later in the song, Belle shows the book to a flock of sheep and we get a look at an illustration that looks nothing like Agrabah:
Unless this book is a compilation of stories, there's no way this is about Aladdin, and given that she refers to it as having chapters in the lyrics, that seems pretty unlikely.
The song does make a pretty straightforward reference to Jack and the Beanstalk, when Belle tells the baker she just finised a story (in a different book) about "a beanstalk and an ogre," so hold tight to the knowledge that other fairy tales do exist in Beauty and the Beast, even if they're not Aladdin.