Detective fiction writers Agatha Christie and Freeman Wills Crofts both had popular successes in the mid-1920s. Crofts introduced his sleuth, Inspector Joseph French of Scotland Yard in this 1925 book. Already in his 50s (he refers to his eldest child having been killed in World War 1), French proved to be so popular that Crofts included him in all his detective fiction for a further thirty years.
The book's title probably continues to attract first time readers to Crofts' work. Readers who like to sample books from the "Golden Age of British Detective Fiction (1920-1940) will find strengths and weaknesses. The book displays Crofts' "puzzle solving" formula admirably. A problem occurs, a theory is formulted, testing follows, each discovery likely to form a "spring board" to further discovery. If a dead-end is encountered, another theory is formulated, etc. Crofts also keeps us in company with Inspector French throughout the whole book.
If these are some of the strengths, then a few weaknesses must be acknowledged. Expect old-fashioned crimes and old-fashioned criminals. The crime and murder here, popular in detective fiction of the time, involved the theft of diamonds. One of the criminal's skills, also popular at the time, was the devising and use of a code. Both of these elements will appear dated and quaint to C21st readers.
So is it Inspector French's greatest case? Reading the thirty or so other books in which he features will give you the answer, together with many hours of enjoyment.