I think that sketching is an art unto itself. Being able to sketch a person or action quickly and with detail is difficult to achieve. Therefore I must point your attention to António Jorge Gonçalves' website ++Subway Life++ where he sketches images of people from the subway. Not only are the sketches interesting, but the site is very creative. It's been around for a while, so check it out—there might be a book available if you are looking for a more tangible means of viewing these.