On the eve of the centennial of his death: an ardent life of Robert Louis Stevenson (1850-94) that emphasizes the man rather than his writings. In fact, Bell (former literary editor of the Scotsman) says little about Stevenson's work--The Strange Case of Dr. Jekyll and Mr. Hyde the one exception--beyond situating it in a career bounded by illness, genius, and the lure of the unknown. But Stevenson as eccentric, rake, invalid, and expatriate comes to light here in vivid coral colors. Fundamentally, Stevenson remains inexplicable: a self-indulgent, flighty man (``as tightly strung as an overtuned piano'') who proved his inner steel by writing through a barrage of illnesses--sciatica, tuberculosis, etc.--that would have crippled others. A religious fanatic as a child, he became an atheist and socialist. A terrible student, he turned into a superb analyst of the human psyche. In Bell's agile hands, Stevenson's life seems a study in how not to do it: He never had a steady home; he married a neurotic woman who exacerbated his own problems; ran away from his literary genius (wasting his time on lesser projects and obsessive rewriting); and perfected his style only during his final years on Western Samoa, writing books he never had time to finish. Yet a few of Stevenson's tales--Jekyll and Hyde, Treasure Island, Kidnapped--will endure, along with the still-accreting legend of the dissolute vagabond genius (which Bell somewhat punctures, maintaining that ``R.L.S. was debauched on a part-time basis only''). A lively biography that Stevenson himself would have enjoyed.