Theydon occupies Flat 18 on the top floor, opposite the titular 17. Much to his surprise the man seen outside enters the building. By the sound of his footsteps Theydon deduces the stranger visits 17, the home of Edith, widow of Arthur Lester, for about five minutes, and then leaves.
Next day Theydon is off to dine with millionaire James Creighton Forbes to talk about the latter's campaign urging the peaceful use of airships. Not long before this engagement Theydon learns Mrs Lester was murdered the previous night. Yet another shock awaits him: Forbes turns out to be the man he saw outside the block of flats. And what's more, Theydon is followed home from Forbes' house by a mysterious grey car.
After being interviewed that evening by Chief Inspector James Leander Winter and Detective Inspector Charles Furneaux, an unlikely pair who appear to be constantly at each other's throats but are *far* shrewder than they seem, Theydon becomes involved willy nilly in their investigation of the murder and its ramifications.
And so the reader is off to the races with numerous twists and turns involving, among other things, an American tourist, a motorcycle chase, kidnappings, shots through windows, pea-sized ivory skulls -- and the return of the grey car.
My verdict: Number 17 might best be characterised as a mystery-thriller with a dash of romance. I liked the sparring police partners, straightforward Winter and imaginative Furneaux, who pop up unexpectedly all over the landscape. There's plenty of action and a bit of suspense and in fact as Furneaux remarks at one point, "Oh, it's a plot and a half, I can assure you", and indeed it is, Oscar, it is.