As a Thief in the Night by R. Austin Freeman

Harold Monkhouse is usually such an uncomplaining patient so when his brother Amos calls in one night, what he doesn't expect is to see him at death's door. Suspicions aroused, he demands an urgent second opinion. And when Harold is later found dead from arsenic poisoning, Amos is left in no doubt that foul play is afoot. The inquiry begins and Barbara Monkhouse is soon singled out as the prime suspect. What ensues is a roller coaster ride into crime fiction at its best as the truth of the fateful night eludes even the best of detective minds. Could it be a simple case of wife poisoning husband-or is it just possible that another shadowy figure stole into Harold's room, as a thief in the night, to rid the world of an innocent man?