In checking out the comments to my post Hail to the Headless Horse! about Paul Berna's 100 Million Francs, otherwise known as The Horse Without a Head, you will see that I made the astonishing discovery that there was a sequel to that novel.
In fact, if the list of information given in this great review of The Horse Without a Head is correct, there are three sequels to the original! The reviewer on "Freaky Trigger" briefly discusses the plot of the second book, which takes place two years later. I found this plot description of The Mystery of Saint-Salgue on e-Bay (cover picture here): "Gaby and his gang of friends have acquired a racketty old Citroen which holds all ten of them as well as eleven stray dogs. In it they set off for a camping holiday in the South of France. During the first night out they meet two strangers from Canada who charm them with talk about a village called Saint-Salgue from which the Canadians believe their home in the prairie took its name. The gang soon realises that the name Saint-Salgue has a mysterious significance not only for their new-found friends but also for two unfriendly characters who are trailing their van." I have not found any information about the fourth book.
A Hundred Million Francs (Le Cheval sans tête), 1955
The Street Musician (Le Piano à bretelle), 1956
The Mystery of Saint Salgue (La Piste du souvenir), 1962
Gaby and the New Money Fraud, 1971
There also appears to be at least two books about Inspector Sinet, the police officer who helps the children in Horse Without a Head:
The Mule on the Motorway aka The Mule on the Espressway (Le Commissaire Sinet et le mystère de l'autoroute du sud), 1967
A Truckload of Rice (Le Commissaire Sinet et le mystère des poissons rouges), 1968
Anyway, majorly cool!