The French answer to the Famous Five and Secret Seven is a series called Les Six Compagnons. From 1961 to 1978, Paul-Jacques Bonzon told the adventures of a group of friends from Lyon. In total there were 38 books published.
The group consists of Tidou who acts as the the narrator, Gorget, Gnafron, La Guille, Bistèque and Le Tondu. There is also a girl named Mady, who is very intuitive and helps them solve mysteries, as well as Tidou’s faithful dog Kafi.
As a child, I was crazy about those stories. I would wait eagerly for the next book and read each of them at least five times. I could not relate to the Famous Five as they came from quite a wealthy background. However, Les Six Compagnons were a group of working-class kids from Lyon so I totally related to them.
I still have all the books sitting on my bookshelves at home. The pages have gone yellow with time, but I still remember the hours of fun that they provided me with. The novels are well constructed, the stories exciting and the style eloquent.
If you are learning French but are not fluent or confident enough to read more complex novels, I highly recommend that you read Les Six Compagnons. Research in second language acquisition shows that reading books at a slightly lower level (such as children or teenage books), is more effective in helping improvement than reading more difficult novels and that series are particularly helpful.