Erik Dietrich (of the Expert Beginner fame) published another great blog post explaining when and why you should write a book. For the attention-challenged here’s my CliffNotes version:

• Realize you have no idea what you’re doing (see also: Dunning-Kruger effect)
• Figure out why you’d want to spend a significant amount of your time on a major project like book writing;
• It will take longer (and will be more expensive) than you expect even when considering Hofstadter’s law.