I've written a book on using Cubase with a general grounding in music, recording and music technology [1]. Income has been very patchy, and never what I'd hoped for at any point. Best month has been around £450, but that's only happened a couple of times (I don't know why, either, unfortunately), usually it's more like £40. Made about £2000 in total, so definitely not something to retire on. Unsure as to whether it's the market for the book, the quality of the book (I hope not and don't think so given the positive feedback I have had from those who have bought it), or a total lack of marketing that's at fault. Not entirely passive as I've had to update it for each revision of the software that has come along, which takes around a week's full time work.

[1]http://www.lulu.com/shop/darren-jones/the-complete-guide-to-...