Clouds of Witness by Dorothy L Sayers

He is dead, and by my hand. It were better that I were dead myself, for the guilty wretch I am. - Adventures of Sexton Blake

Hour after hour Mr. Parker sat waiting for his friend's return. Again and again he went over the Riddlesdale Case, checking his notes here, amplifying them there, involving his tired brain in speculations of the most fantastic kind. He wandered about the room, taking down here and there a book from the shelves, strumming a few unskillful bars upon the piano, glancing through the weeklies, fidgeting restlessly. At length he selected a volume from the criminological section of the bookshelves, and forced himself to read with attention that most fascinating and dramatic of poison trials -- the Seddon Case. Gradually the mystery gripped him, as it invariably did, and it was with a start of astonishment that he looked up at a long and vigorous whirring of the doorbell, to find that it was already long past midnight.