The plot of the story involves a lost cow and the subsequent search for it's location. The story has a rigid structure, in the pattern that follows:
Is that my cow? It goes "HRUUUGH!" It is a hippopotamus! ... No, that most certainly is not my cow.
This goes on for a fair while.
Sam Vimes has an unbreakable rule, first mentioned in Thud! of always reading Where's My Cow? to Young Sam. This (spoilers!) becomes very important by the end of the book where, though being possessed by the Summoning Dark and battling deep-down dwarfs, he screams the words so loudly that his son can hear them from miles away. After the publication of Thud!, an actual version was made available, complete with Vimes' Ankh-Morpork annotations
On the cover, it has the badge of the Ankh Morpork Librarian's Award.