A text editing app I wrote for Gary, with two main tables - ppt (people, places, things - the way he liked it, and it made a lot of sense) which would hold the articles, and then a hierarchy table which would hold them linked into books. With a sufficient number of semi-independent articles (which is quite possible in his area, astrology) you could churn dozens of books with same texts ordered this way or other, without writing too much of new stuff.
Then, some day in may 2007, about the time I got out of debt, I had the idea that I could use it to write... this. It gradually morphed into the Byo project, where I removed most of the hierarchy manipulation stuff but added this semiautomatic linking.
It's august 2019 now, about the time I got out of work, and I'm still writing it.