Go freelance. Tell your boss you’ll do the work for a third of what he’s paying you now (and with no benefits), but you want the right to publish and sell the material on your own.
I would if this were a publicly available product, but it’s an in-house software system.
However, we recently bought software from another company and the documentation was junk. I rewrote it so we could use the system, and I played with the idea of reselling the documentation back to them to replace the junk they use now. I’ll talk to my boss and see what he says. Thanks!