The People Spotters Guide Book, a field guide of one of the most fascinating species that wander around on this earth; Your!
The pocket book contains 50 different types of people divided into different chapters, with everything from people who pick their nose publicly called "The Nose-Picking Commuter" and people on Segway aka. "Ambulophobes" to people who use socks in their slippers aka. "Socks in Crocs". Each type is presented with a short text in English, a couple of examples photos in the true seen-on-internet spirit, as well as a box where you can fill in your own observations. Just like bird watching, but in the city.
If you do not want to go out on "human watching" yourself, then of course the book is at least as fun just as it is, watched in at home.