Strong Poison by Dorothy L Sayers

Lord Peter congratulated Miss Murchison and gave her a rather special lunch at Rules, where there is a particularly fine old Cognac for those that appreciate such things. Indeed, Miss Murchison was a little late in returning to Mr. Urquhart's office, and in her haste omitted to hand back the skeleton keys. But when the wine is good and the company agreeable one cannot always think of everything.

Wimsey himself, by a great act of self control, had returned to his own flat to think, instead of bolting away to Halloway Gaol. Although it was a work of charity and necessity to keep up the spirits of the prisoner (it was in this way that he excused his almost daily visits) he could not disguise from himself that it would be even more useful and charitable to get her innocence proved. So far, he had not made much real progress.