A Prince of the Captivity by John Buchan

Adam Melfort began his new life in a kind of daze. The stone walls which made his prison did not circumscribe him, for he was living in a far narrower enclosure of the mind. The dismal fare, the monotonous routine were scarcely noticed: he was allowed books, but he never opened them; visitors were permitted on certain days, but he did not welcome them, and the few who came -- a cousin, a brother officer or two, Christopher Stannix -- found a man who seemed to have lost interest in the outer world and had no need of consolation. His wife was not among these visitors.

The truth was, that ever since the tragedy Adam's mind had been busied with a problem of conduct. He believed that he had acted rightly, but doubt intervened with maddening iteration, and a thousand times he had to set the facts in order and review his decision.