It’s an excellent movie. Also well worth reading the novel, by John Buchan. It’s the first in a series of novels he wrote with the same characters, in connection with the First World War.
It's a superb novel, and still holds up today. I've only read one of the sequels, Greenmantle, but I wasn't impressed. Richard Hanay, Agent of the British Military Intelligence, is a much less interesting character than Richard Hanay, wanted man on the run from both the police and the spies.