Where's My Cow? - Terry Pratchett

A father starts adapting his son's favorite book about farm animals to fit life in a city environment. Instead of moo-cows and baa-lambs, he weaves a tale about streets, not fields, and about rogues, villains, and the place his son will grow up.This is a book about reading a book, which turns into a different book.But it all ends happily!