Writing books which generate revenues of about $500 per month (all of them combined, not every single one). See http://t3x.org
Same goes for the Klong book, which I bought and enjoyed thoroughly. Haven't mastered the language, though, but I'm hoping to; it definitely is really interesting, especially for a person like me who loves the build-your-own-tiny-tools-with-tiny-languages-for-your-own-use-and-prefer-low-end-computers philosophy. I'm definitely a fan of your work; your webpage and entire way of life has been an inspiration for me for years already.
If you don't mind me asking -- are you working on any upcoming books these days as well?
At the beginning writing somehow fascinated me, but right now it is probably just habit/momentum/karma that keeps me going.