Night Ferry to Death — Patricia Moyes

As passengers board the overnight ferry from the Netherlands to England, a desperate little man frantically pleads for a cabin as if his life depended on it. But the ship is overbooked, and his only choice is to sleep in a lounge with everyone else, including Chief Superintendent of Scotland Yard Henry Tibbett and his wife, Emmy, who are returning from a holiday in Amsterdam.

The next morning, the man is found dead -- the victim of a killer who had to be one of the Tibbetts' sleeping companions in the lounge. Furthermore, the dead man turns out to have been carrying a fortune in stolen diamonds, which have disappeared. When Emmy makes an astounding discovery about the contents of her overnight bag, her life is put into grave danger as the Tibbetts are enmeshed in a sinister web of international intrigue that leads them to take one last trip on the night ferry, a voyage that proves even more shocking than the first.