More comments / suggestions. The page on stone layers looks brilliant!
A page on moods -- Yeah, it's been done to death, but no compendium would be complete without it. Possible discussion of "controlling" moods: Turning useless migrants into platinum warhammers just waiting to happen, forbidding materials to force a better price/quality, that kind of thing.
A page on mandates -- Avoiding nobles that issue bad ones, when & how to satisfy those you get, discussion of luxurious jail cells, giving a wiffle bat to the Captain of the Guard.
A page on Evil weather and its effects.
A page on blood -- Why rain both creates & destroys it, where dwarves will (and won't) clean it up, how to get rid of it, & why it's important to do so before it spirals out of control. If you think, "Ah, that's just an aesthetic point, my fort looks cooler covered in blood anyway," just replace the word "blood" with "syndrome-inducing Forgotten Beast extract" and you'll see why you should have practiced when it was just blood. Discussion of wells (and keeping them clean), Dwarven Bathtubs, Dwarven Showers.
A page on save-scumming -- What it can & can't undo. Reloading a save made during a mood can change the resulting artifact, but not its name. Reloading a save made prior to a baby's birth will re-randomize everything about the baby. Reloading a save made after starting a new season will not change the dates that merchants, goblins, or megabeasts enter the map during that season, but it can change the positions from which they enter, the type of goblins (ambush vs. siege), and the specific megabeast.
Possibly, a page on self-imposed rules: Is it ethical to pretend that fire-breathing dragons cannot escape flimsy wooden cages? Is it an abuse to run a Dwarven Water Reactor to produce free energy from nothing? Are picks overpowered as weapons? Etc.
As PDF urist master says, daggers and especially whips are important to document, especially due to their amazing lethality (are whips fixed now?). Also, I'm surprised you left picks off your list of weapons, seeing as how they're practically the dwarven equivalent of a lightsaber.