Pet Rock was a joke book that happened to come with a rock.
In 1975 Gary Dahl joked to friends in a Los Gatos bar that his only pet was a rock. He spent two months writing it a spoof dog-training manual, then learned a book with no agent rarely reaches a shelf. So he picked the rocks himself and boxed one with every manual. About 1.5 million sold at $3.95.
Pet Rock on Brandseum