While this is fun if you like writing, it's not really a reliable or efficient source of income.

It takes me about 6 to 10 months to write a tech book in my spare time, and unless one hits the popularity jackpot at some point, I don't think they will pay off financially.

(But published books do make a very nice line on a CV)

I do this. It's hard to grow at first and there's plenty of ongoing marketing work but after 2 years I could already go full time if I didn't insist on living in SF.

It's a fun little cottage industry.