Parnassus On Wheels is written in the first person, in the voice of Miss Helen McGill, a 39 year old spinster. She keeps house for her older brother, Andrew, who is- or rather, had been- a businessman. One day some years before, Andrew decided to leave his job, bought a farm, and moved to the country. Helen accompanied him, and was quite content on the farm until the day her brother decides he's an author and writes a book on the wonders of the rural life, called Paradise Regained. Surprisingly his book becomes a bestseller, and Andrew becomes a bit of a celebrity. He begins to devote more and more time to writing and less and less time working on the farm. He also begins having literary friends come stay, getting underfoot and distracting him even more from the running of the farm. Worse, Andrew also decides that he needs to travel about the countryside, camping in the great outdoors so as to write more authentically about it, and so is frequently away. Now saddled with almost all the responsibility of keeping the farm going while her brother writes about how wonderful farming is, Helen is getting pretty fed up. The final straw occurs one morning when Helen, working in the kitchen, looks up to see a horse drawn wagon pulling into the yard.
This marks the beginning of Helen and Roger's adventures on the road. At first awkward about making sales, Helen learns from Roger's expertise and soon becomes enthusiastic about supplying books to those who need and want them. But the two of them have other adventures as well, up to and including dealing with nefarious wagon thieves. In addition, Andrew is hot on their trail, angry and alarmed, convinced that his sister has lost her mind.
The book is not a long one, and the story wraps up quickly, amusingly, and with a dash of romance thrown in.