Zeiler's ECCV14 paper is good follow on reading from this. He was one of the first people to actually visualise what these networks were doing.

https://www.cs.nyu.edu/~fergus/papers/zeilerECCV2014.pdf

Have a look at Figure 2.9. What you see is that the upper layers tend to be very general, basic filters that act as edge, corner and circle detectors. When you move to lower levels, the filters get more complicated and you start seeing things like animal parts, faces and so on.