Aside from my full time career as a programmer, the best side income stream I've built has been my book I wrote[0] to help software engineers develop the soft-skills that are necessary to reach a senior level position.

I spent 4 years on and off writing and preparing the book for publication, and it's been bringing in a steady stream of income since it was published in October 2022.

[0]: https://www.holloway.com/b/junior-to-senior