Speaking about required reading: head over to sleepycity.net for wonderful (and long) write-up by a guy who has spend the last years exploring (at night on foot) the Paris Métro system. The article is illustrated with a bunch of terrific photos.
Reading this makes me jealous. Personally i have never come any further than maybe 150 meters into the tunnel extending from the ‘Waterloo‘ station in my old hometown of Hannover. These guys claim that they have explored alomst the entire Métro system in Partis (214KM according to wikipedia). respect!