I'd not come across 'urban exploration', or 'urbex', until Robert Macfarlane did a story in The Guardian ( http://www.theguardian.com/books/2013/sep/20/urban-exploration-robert-ma... ). I was so intrigued, I picked up a copy of this very striking and fascinating book. Garrett is an American photographer and researcher living in England. He became drawn to the loose groupings of nocturnal explorers who semi-illegally enter urban spaces, attempting to experience cities in a different and unique way. They enter everything from bridge structures, unfinished high-rise buildings, sewers, underground railway systems and abandoned industrial ruins to explore, document, and reclaim the cities. Mostly it's in London, but they also travel and meet up with similar groups in Europe and the US.
This is quite a scholarly book, as Garrett is researching this phenomenon, as well as participating. This does make some of his claims about the importance of what they are doing seem a little grandiose, but at heart it's a great adventure in which they are always in danger of arrest, and the photography is stunning. They get to experience the city in a way that I never will!