This blog post is worth reading. In it he apologizes for the book taking so long and expresses some regret for taking on the project in the first place:

"Ironically working on the book (or not working on it, however you look at it) actually compelled me to NOT blog more as every time I wanted to write a technical blog post I was forcing myself to make the decision “I’m writing about 1000 words on a technical matter, shouldn’t this just be going towards my uncompleted book?” and would just end up writing nothing as a result."