Quality of Service is one of those topics that has always peaked my interest. You don't see it deployed in every network, nor is it really necessary if the bandwidth never approaches what the network is capable of supporting. However, many situations call for traffic queuing and shaping behavior beyond the default configuration - that is where good design comes in.
The Cisco Enterprise QoS Solution Reference Network Design Guide is one of the best reference guides I've ever seen for configuring QoS on Cisco equipment. There are books and other white papers on QoS, but those either cost money or make you sign up for an account to access them. This one is free.