By any measure, that is some serious typing. I've written numerous emails, memos and blog posts on my phone, but those are 200-1,000 words at best. Believe it or not, I rarely use the slide-out thumboard on my device, relying instead on an input method called Fitaly, where I can achieve around 40-45 words per minute. I do that mainly because my thumbs tend to get sore after too much typing, and it seems to be too tedious for me. Different people have different preferences for input methods on mobile devices. Thumb board are popular because if you can touch-type, you at least know exactly where the keys are located, so the learning curve is low. My first experience with data input on a mobile device was an on-screen QWERTY keyboard designed to be used with a stylus, which I found abysmal. I decided to learn Fitaly for that reason.
I know others that fell in love with Palm's Graffiti. In fact, Jeff Kirvin, another author, wrote a book on his Palm years ago using nothing but Graffiti. I also know a few people that like on-screen handwriting recognition, like Windows Mobile's Transcriber. Whatever you use, you'd need to be very comfortable with it to use it for two years, morning and afternoon, to write a book with it.
Given the cost where mobile technology was a few years ago when Mr. Brett started, using a phone as his primary device is understandable. Today, I'd buy a $350 netbook in a heartbeat for the same task.
If you are interested in picking up the book, you may find it at your local book store, or you can read the synopsis at eReader.com and download it to your mobile phone. Amazon has both the hardcover and Kindle versions available.