A big fan of Harry Beck, and a regular tube-user, I bought this after agreeing to go with a friend to a U3A lecture by the author.
It's expensive for a skinny book but good although occasionally heavy going with Dr Roberts being an academic (an 'ologist' not an engineer or transport planner). Also slightly nerdy, at least compared to 'Mr. Beck's Underground Map' by Ken Garland which I enjoyed more.
That said, it will probably appeal to the tuboholics out there and to train spotters, of which there are plenty. Hence the 4-star rating: it's definitely one for the boys.