I bought this book in beta and used the information to launch http://os-blog.com. Probably the best testimonial I can give is telling you about my own success thus far: It's been a little under a month and analytics has the site pegged at ~30k unique visitors (thanks in no small part to HN).

The book won't transform you into a blogging sensation overnight, but it does have a lot of good information on technical blogging. If you're thinking about blogging about technical things, you should buy it.