I think I would have rated this much higher if it weren't for the title. If this was Inspector French's Greatest Case, then it's no wonder that his books are virtually forgotten today.
A man goes to work at a jewelry shop only to find the dead body of a senior clerk and an empty safe. French takes over the case. The investigation is described in mind-numbing detail, along with his frequent trips to Europe - the Netherlands, Spain, France, etc, which are either by rail (with the entire route carefully described, including a time table) or by sea. At this point, I had all but given up. So when I temporarily misplaced this book, I wasn't exactly heartbroken. And when I got it back, I finished more because I wanted to get it over with than because I really cared how it ended.
It ended with a big climax aboard another ocean-going ship, complete route included (just in case you wanted to book passage) and Inspector French being surprised by the identity of the murderer.
This was a classic example of "tell, don't show." Not recommended for anyone. If you want to try another book by this author (this was his first book, and it really shows) Inspector French and Cheyne Mystery is better. Although even then, the ending could have been much better. Crofts seems to go for the very conventional story.
To be fair, this might have been a much better read at the time. But as a modern reader, I kept thinking that if he had taken a plane ride, it would have sped up the plot a lot. And where was Scotland Yard getting all this money to pay for his fares? Never once does French end up short on cash or miss his connection. Silly read, all around.