Petzold writes an interesting blog on authoring programming books. I agree 100% with him, you do not do it for the money. I co-authored a book once and did not receive a dime, not one dime, for my efforts. In fact, I get paid more to do a technical book...
http://www.microsoft.com/downloads/details.aspx?familyid=69d2219f-ce82-46a5-8aec-072bd4bb955e&displaylang=en
http://www.retrospector.com/2006/06/21/top-10-ways-to-motivate-geeks/