For The Defence: Dr. Thorndyke (Dr. Thorndyke Mysteries #23)
R. Austin Freeman

Andrew witnesses a man being shot one night by two masked assailants and on the advice of his wife, decides to remain silent about what he has seen.
But when a meeting with his cousin Ronald proves to be fatal, Andrew finds himself suspecting foul play, as Ronald lies crushed on beach sands. And soon, Andrew is a suspect himself.