I'd like to see this and more variation in generated "non-fiction" too. Historical figures like necromancers and megabeasts already write historical stuff, but what would really be neat would be to see some scientific texts like the works of Theophraestus and Hippocrates. Just for the chance of finding a detailed treatise on the life-cycle of plump helmets or a list of medicinal uses for golden salve.
Geneology books could probably be written by the game in their entirety, and I could see them being useful for things like tracking down the heir to the throne (or night-creature children that can only be killed by the bones of their ancestors).
Bestiaries and histories could be done too in a cruder sort of way by copying over the game descriptions and Legends mode.
Account books would be great for players who want to be merchants, bandits or pirates - they'd also be the sort of thing I'd like to see generated in Dwarf Mode, with brokers and book-keepers keeping books of accounts to store trade treaties and inventories respectively. These should get filled up over time and have to be stored in cabinets, chests or a new bookcase furniture item. It'd be neat if Adventurers could visit your dead fortress and snoop through your inventory.
Another neat idea would be a Chronicle of your fort, maybe made up of dated Reports, that an adventurer could read through later.
Religious books need to be a thing, too, depending on the religion.