The first half of this is a history of the study of Stonehenge, which is fairly interesting.
The second half is really interesting. Johnson decided to work out how the builders designed Stonehenge, and this book describes his attempts to work out how the various parts were positioned, using only techniques that would have been available at the time (i.e. pegs and ropes). Pretty cool stuff.
On top of that, it's a coffee table book with lots of nice pictures.