I co-authored a technical book about 15 years ago. The advice I got from another author was that you "can't get rich writing books", and it's not worth it for the money.

Since then, book sales have declined... particularly technical books. I wonder if things are now also worse than the experiences of other commenters? For example, David Flanagan, O'Reilly's star author on Java and Javascript, switched back to consulting because even he couldn't make enough money.

All that said, it's great to be a published author! I couldn't help but smile when I re-read some of my work recently.