I have this book 'transit maps of the world' with the subway systems of cities around the world. Its interesting to see how some smaller cities have relatively elaborate systems and some major cities have very little.
https://www.amazon.com/Transit-Maps-World-Worlds-Collection/dp/0143112651
That was a cool slideshow. Did anyone see the pictures of the Swedish subway system that were on Reddit a month or two ago? Also, has anyone else checked this book out? I snagged a cheap used copy. It definitely focuses on the design aspect of the map, rather than tons of info about the subway, but it'll make any transit geek happy.
I'm all about transit maps. I recently went into an Amazon store and found a coffee table book which is basically the /r/gonewild of transit maps.
I bought the book. Looks good too. http://www.amazon.com/Transit-Maps-World-Mark-Ovenden/dp/0143112651
If you like these, you should probably have a look at Transit Maps of the World which contains a wealth of information on how these maps were designed and evolved.