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.