We are about to embark on our own side extension and, because of our previous experience in trusting "the experts", have decided to act as our own build managers on this job.
(Our previous experience was a barn conversion, and were ripped off royally as we were foolish enough to appoint a build manager who simply employed just one subcontractor (a friend of his) to do all the work, so no chance to compare quotes and get the best deals).
We then had to spend a lot of time and effort rectifying problems caused by incompetent building work (flooding in the lounge for example because of the wrong dpm being used for cheapness). The build manager also pocketed the fees we paid him to pay for an NHBC guarantee, so we had all that to sort out as well.
This book is a mine of information, from the planning to the final completion, and covers all the stages in between - which is useful to get a feel of how a build actually progresses.
You also get the benefit of knowing what you are talking about as he provides a lot of detailed information on each of the components that should be used to get the job done properly. He points out, for example, that some builders will try and omit the cavity tray on an extension because it is a fiddly and time consuming job for them (but, of course is essential for you if you want a dry room at the end of a rainstorm).
It is a very useful source of reference, and I just wish that I had bought a book like this before, so that at least I could have argued over certain issues, rather than just being presented with a huge bill and an ocean of problems to rectify.