I wrote one last year. It started out as notes for a series of "lunch and learns" I was going to give at work. The notes just kept growing and growing, until over the course of a weekend I ended up sitting down and pounding out 200 pages of content. That was the easy part! The hard part was then learning how to build an effective index (harder than it sounds), coming up with examples and figures, tuning and tweaking, etc.

So, to recap my experience, I would suggest that you think of a topic about which you know enough to have something to say. Not just "pure knowledge," but real opinions founded in real-world experience. Pretend you're going to give a series of presentations, lectures or articles on the matter. Start typing up the notes, looking for organizational structure as you go. Keep typing. Reorganize as you go. Keep typing. At some point you'll realize you're on your way, then the momentum will start to carry you as you figure out to what "depth" you want to go. Don't edit at this point, just keep generating content.

When you think you're close, start "spiraling" back through the book, editing, cleaning, editing, adding, editing...If the material deserves an index, go read up on that (again, it is NOT just a cross-reference of non-trivial words!) Somewhere around now hand it out to some friends to read. Incorporate their feedback. Edit again.

Look! You've written a book! :)