Agaton Sax and the Diamond Thieves

Agaton Sax and the Diamond Thieves (Agaton Sax och de slipade diamanttjuvarna; published in English in 1965) is a book about detective Agaton Sax by Swedish author Nils-Olof Franzen. It was the first of the series to be published in English.

Plot summary
The Koh-Mi-Nor diamond is stolen and a very clever thief is putting messages about it, in a secret code, in the personal column of the newspaper published by the Swedish detective Agaton Sax.