Easy answer is at £16 from Amazon you can't go wrong. There is stuff in it that most architects wont know but it's not about Passivhaus window details or hardcore technical stuff. However it's not the usual coffee table Grand Designs type book that leaves you hungry (but there are great pics). I'm sure us techies will find stuff we disagree with but Jon has built his own house(s) and has several(!) years experience with self builders and big architecture alike, he's at the coal face.
I'm enjoying it on many levels including as a record of UK eco-self build history. Interesting to see the evolution and common threads.
I'm trying hard to be dispassionate about it, but for me, and I'm sure for others who have been in this field a while, its a very evocative book. If you don't like it I'll send you your money back (that's just Mark not the entire internet).