Sigulbeard: you are assuming all NPC's would be literate. Right now, only demons, necromancers, cities (and a sometimes deities) create text to be read in the form of slabs and books. This argues for a very limited level of literacy and would keep even fortress mode writers writing about things that were important to them: bookkeepers keeping up to date lists of supplies, writings about artifacts (including other books) and things like a new mayor coming into power or a goblin invasion killing half the fort.
Paper-making and book binding should not be a trivial process either. The production of ink/paints, paper/vellum, binding glue, pens/brushes and appropriately sized book covers, not even including all the decorations usually applied to such objects would be an industry and skill set all on its own. Once all that is available, I imagine that each book itself would be a masterwork artifact with a random number of pages, dedicated to an event in Legends mode.
Imagine porting the entire list of "The Terrible Battles" from Legends mode into a book, titled "The Terrible Battles". The memory is already taken by the world gen, the book would just be a call to that portion of the text already stored. Reading the book would read that portion of Legends mode. New books produced by adventurers would be stored in the history of Legends mode, allowing them to be "read" from that source. Suddenly, "Was it the Dwarf?" makes sense as a title, because it references a dwarven individual who at one time "explored the depths of the world and tamed the naked moledogs" before becoming a general and starting "The War of Shining" against a neighboring goblin civilization.
UPDATE:
After thinking about this for awhile, my idea seems more and more doable. The number of pages would be generated, which would allow for that number multiplied by, say, five to eight consecutive entries in Legends mode per "page". After this, one of the categories from Legends mode would be chosen "randomly", and the title of the next subset would be the one referenced in the book's title. Starting with the first entry then reading downward consecutively, we'd have the contents of the book, (or in the case of the maps, the map would appear with a specific portion blinking to show "special interest". I figure the map version shows the location of one of the Megabeast lairs or other high level beastie, which would unhide it on the quest screen as well as some title saying, "Beware all ye who travel, for XX makes their home here"). For more fun yet, we could add "part 2" to a title that has already been generated and add the number of pages of the first volume to the total before beginning the contents, so books with less pages than the content allows for could just be the first in a series to catalog that particular aspect of the world's history.
Literate adventurers could set up journals as "margin notes" in any of these types of books, either using blank pages from a book that had too many pages and too few entrees to fill them or adding commentary like "1091: Dinnerwandered almost completely deserted, except for the vampires, zombies, kobolds and goblins..." (this one sounds a bit more difficult to implement, though, since it would require an input interface for the player.)