The Man in Lower Ten

"The Man in Lower Ten" is a mystery by Mary Roberts Rinehart.

Excerpt from the book:
McKnight is gradually taking over the criminal end of the business. I never liked it, and since the strange case of the man in lower ten, I have been a bit squeamish. Given a case like that, where you can build up a network of clues that absolutely incriminate three entirely different people, only one of whom can be guilty, and your faith in circumstantial evidence dies of overcrowding. McKnight could tell the story a great deal better than I, although he cannot spell three consecutive words correctly. But, while he has imagination and humor, he is lazy. "It didn’t happen to me, anyhow," he protested, when I put it up to him. "Besides, you want the unvarnished and ungarnished truth, and I’m no hand for that. I’m a lawyer." So am I, although that has been disputed. I am unmarried, fond of outdoors, and completely ruled and frequently routed by my housekeeper, an elderly widow.