I saw this great post on the nuts and bolts of writing a book on the BJUG mailing list. Well worth a read.
I have sympathy in particular with his ‘writing schedule’ comment. I started to put together a book with some friends, and it was hard to keep things moving. We ended up not moving forward with the book and placing the content on a blog (which has proven hard to update as well). The book was about software contracting.