Interestingly, what Twilight tends to read in her offtime is mostly left up to the readers' imagination. She's mostly seen with nonfiction books, but that's usually because they're relevant to the plot.
She does canonically have an affection for the Daring Do series, which is essentially Pony Indiana Jones: The Book Series. So we can assume she appreciates the adventure genre, which isn't exactly surprising. But beyond that, we can assume she appreciates fantasy, which would be almost like nonfiction or historical fiction for her given the sort of world MLP takes place in, and historical pieces, be they fiction or nonfiction. Really, the only genre I can see her actively disliking is romance, because it's a combination of a subject she's almost completely uninterested in, and characters who she can almost never relate to. At all.
no subject
She does canonically have an affection for the Daring Do series, which is essentially Pony Indiana Jones: The Book Series. So we can assume she appreciates the adventure genre, which isn't exactly surprising. But beyond that, we can assume she appreciates fantasy, which would be almost like nonfiction or historical fiction for her given the sort of world MLP takes place in, and historical pieces, be they fiction or nonfiction. Really, the only genre I can see her actively disliking is romance, because it's a combination of a subject she's almost completely uninterested in, and characters who she can almost never relate to. At all.