Into the Blue by Robert Goddard

Harry Barnett is a
middle-aged failure, leading a shabby existence in the shadow of a past
disgrace, reduced to caretaking a friend’s villa on the island of Rhodes and
working in a bar to earn his keep. Then a guest at the villa–a young woman
he had instantly and innocently warmed to–disappears on a mountain peak.
Under suspicion of her murder, Harry stumbles on a set of photographs taken
by Heather Mallender in the weeks before her disappearance. Desperately,
obsessed by the mystery that has changed his life, he begins to trace back
the movements and encounters that led to the moment when she vanished into
the blue. The trail leads him back to England, to a world he thought he had
left for ever, and at every step of the way a new and baffling light is shed
on all the assumptions that have made Harry what he is.