Christie, Agatha -- The Mysterious Mr Quin (1930)

Blurb: Mr. Satterthwaite is a dried-up elderly little man who has never known romance or adventure himself.  He is a looker-on at life.  But he feels an increasing desire to play a part in the drama of other people—especially is he drawn to mysteries of unsolved crime.  And here he has a helper—the mysterious Mr. Quin—the man who appears from nowhere—who “comes and goes” like the invisible Harlequin of old.  Who is Mr. Quin?  No one knows, but he is one who “speaks for the dead who cannot speak for themselves”, and he is also the friend of lovers.  Prompted by his mystic influence, Mr. Satterthwaite plays a real part in life at last, and unravels mysteries that seem incapable of solution.  In Mr. Quin Agatha Christie has created a character as fascinating as Hercule Poirot himself.

See also: http://onlydetect.wordpress.com/2010/04/06/agatha-christie-the-mysterious-mr-quin-1930/