My dad has been writing a specific non fiction book since forever (at the very least 25+ years, possibly longer?). It's not exactly the kind that can be finished. If you need to know about that specific topic, there's that one and like one other that goes into as much depth.

He resurrected and nearly completely rewrote someone else's thing from mid 20th century era, which itself was a resurrection of an late 19th century era thing.

But yes it's exactly as you describe - keeps expanding and updating. Near 6000 pages now?