The 39 Steps by John Buchan

I read this book for the following category in my 2017 Back to the Classics Challenge:

8.  A classic with a number in the title

I thought the book was pretty good, but not great. From Amazon:

The novel is set during May and June 1914; Europe is close to war and spies are everywhere. Richard Hannay has just returned to London from Rhodesia in order to begin a new life, when a freelance spy called Franklin P. Scudder calls on him to ask for help. Scudder reveals to Hannay that he has uncovered a German plot to murder the Greek Premier and steal British plans for the outbreak of war. Scudder claims to be following a ring of German spies called the Black Stone.

I rate it 3 out of 5.