Dangerous Davies: The Last Detective by Leslie Thomas (A Dell Book 1976)

This is the story of a man who became deeply concerned with the unsolved murder of a young girl, committed twenty-five years before.

He was a drunk, lost, laughed at and frequently baffled; poor attributes for a detective. But he was patient too, and dogged. He was called Dangerous Davies (because he was said to be harmless) and was known in the London police as ‘The Last Detective' since he was never dispatched on any assignment unless it was very risky or there was no one else to send.