Walk the Lines which documents his, erm, frankly quite ambitious* [you may prefer to think 'barmy'] attempt to walk the entire length of the London Underground, following the route of the trains, but staying above ground, using the public roads. I'm not a book reviewer, generally speaking, but I'll make an exception for anything quirky and London - and this admirably fulfils both criteria.
The book is, in fact, easy on descriptions of walking and big on London facts and trivia. It is not psychogeography in the tradition of Sinclair or Ackroyd (for which, personally, I was quite thankful) and instead is an engaging stroll/marathon through the metropolis, in the amiable company of the author. Despite the inevitable slog of walking such a distance (sorry, should have made a note of the mileage, but it's a bloody long way), Mason keeps the prose light and entertaining and you enjoy following him on his journey. The book also has the distinction of being (surely) one of the only texts to have pondered the sociological significance of Tottenham Hale Retail Park before it became the first victim of the recent riots (Victoria Line, of course).
The bit I found most engaging was the line I knew best - again, the Victoria - and you may have similar responses, according to how much you know the capital. Some of the best bits are interviews with Londoners who relate to mapping the city - the City of London planner, the taxi-driver, Bill Drummond (ok, you will need to read that one to know why he's relevant) - and I perhaps could have wished for a few more of these; and there is, inevitably, an element of repetition - the later walks seem to lack some of the detail of the earlier (or is that just because I'm not so familiar with the places?). Yet, I very much wanted to follow Mark through to the end, and didn't regret it.
In short, an enjoyable read for the London nerd. If you love London, I defy you not to finish it thinking 'hmm, I wish I had thought of that' and maybe even 'I wish I had done that'.
I don't think my feet could take it, mind.