(French review to be added later)
If you want a mystery series set during the reign of Louis XIII of France, look no further: the adventures of Louis Fronsac are the right ticket.
Louis Fronsac's mystery adventures are actually a great way to learn French history from this 17th century period. The books aren't published chronologically but the author, Jean d'Aillon, recommends that readers follow the internal chronology so we can appreciate the political developments in the background (and I second his suggestion). d'Aillon is an excellent historian and puts an incredible amount of research into every book. Edutainment at its finest.
In this adventure, Louis is 11-12 years and just starting school at the school that will eventually become the famous Lycee Louis le Grand (named after Louis XIV, who isn't king yet). For now, it's school run by the Jesuits, who are on thin ice with the king but maybe on the mend. (Something something, Jesuits murdered the previous king Henry IV. The book explains it better, plus your inner Religious Wars enthusiast would appreciate this detail). Louis gets used to life at school, meets characters who will be part of an excellent recurring cast, and uncovers a mysterious plot to entrap the queen. Yes, Three Musketeers fans, this is the story of the infamous Queen's Jewels given to the flirtiest of flirts, the Duke of Buckingham!
I love that Louis is clearly limited by his status as a child in a very regimented lifestyle. He can't just go being a kid detective, and when he breaks the rules, it's at great personal risk (not to mention the very real and present dangers of gallivanting in 17th century Paris unprotected and sticking your little child nose in the business of men who mean serious, deadly business). I loved every second of the story, whether it was a slice-of-life about a boy finding his place among his peers (and social superiors, because even kids have ranks in this society) or whether it was Louis and his friend Gaston trying to foil a plot way, way above their pay grade (they're unpaid).
Recommended if you love history, adventure, and kid detectives who feel like real kids. 17th century kids, anyway. I'll also note that the book is fairly short and available digitally in the US, untranslated, and the level of French is very manageable if it's a second language you studied in college at... a 400/ 4000 level? Whatever high levels exist today/ used to exist.