Sherlock Holmes, the self-proclaimed world's only consulting detective, is the paragon of clever reasoning, famous for startling leaps of inference. Any of his appurtenances—the deerstalker hat, the pipe, the magnifying glass—is enough to evoke observation and cool-headed rationality. And, despite his singular occupation, he offers a lesson for the rest of us on the uses of scientific thinking in everyday life.

Arthur Conan Doyle draws readers into the process of detection with what his biographer John Dickson Carr called "enigmatic clues." Holmes signposts a piece of evidence as significant but doesn't immediately reveal its use, leaving it as an exercise for the reader. "The creator of Sherlock Holmes invented it," Carr wrote in 1949, "and nobody . . . has ever done it half so well." In one of the most celebrated examples, Sherlock Holmes quizzes a client about the "curious incident of the dog in the night-time." "The dog did nothing in the night-time," the man says. "That was the curious incident," remarks Holmes.

Holmes's supreme rationality is of a piece with his interest in science. "The Scientific Sherlock Holmes," by James O'Brien, an emeritus professor of chemistry at Missouri State University, explores the forensic methods and scientific content in the Holmes canon as well as his creator's own scientific background. Born in 1859, Conan Doyle took to books at the encouragement of his mother. Frustrated by the rigidity of his Catholic schooling, he moved toward science. At 17, he began medical school in Edinburgh. There his mentor was Dr. Joseph Bell, a man with sharpened diagnostic abilities who would serve as a model for Holmes. In one instance, Bell gleaned that a woman who had come in with her child was from the town of Burntisland (her accent), had traveled via Iverleith Row (red clay on shoes), had another child (a too-large jacket on the one present) and worked at a linoleum factory (dermatitis on fingers).

After his schooling, Conan Doyle's rich uncles offered to set him up with wealthy Catholic patients in London, but he sacrificed the opportunity by proclaiming that he was now agnostic. His first tax return, listing an income of £154, was mailed back to him with a note: "Not satisfactory." He returned it with an addendum: "I agree entirely."

Lack of patients gave Conan Doyle time to focus on fiction. His greatest influence was Edgar Allen Poe, who had invented detective stories in the 1840s. Poe's protagonist, C. Auguste Dupin, was, like Holmes, a clever eccentric with a narrator sidekick and a dim police force as his foil. But Conan Doyle saw room to improve: "I had been reading some detective stories, and it struck me what nonsense they were, to put it mildly, because for getting the solution to the mystery the authors always depended on some coincidence." So came his heavy reliance on science, as well as those enigmatic clues. He published the first of his 60 Holmes stories in 1887, and by the third he was boosting sales of the magazine that ran them by 100,000 copies per issue. People were hungry for the stuff.