As a young doctor just starting out, Henry Marsh watched a neurosurgeon operate on a woman’s brain, going after a dangerous aneurysm that could rupture and kill her. This kind of surgery — taking place several inches inside the patient’s head — was perilous, and often compared, as he writes in his riveting new memoir, to bomb disposal work, “though the bravery required is of a different kind as it is the patient’s life that is at risk and not the surgeon’s.”

There was “the chase,” as the surgeon stalked his prey deep within the brain, then “the climax as he caught the aneurysm, trapped it, and obliterated it with a glittering, spring-loaded titanium clip, saving the patient’s life.” More than that, Dr. Marsh goes on, “the operation involved the brain, the mysterious substrate of all thought and feeling, of all that was important in human life — a mystery, it seemed to me, as great as the stars at night and the universe around us. The operation was elegant, delicate, dangerous and full of profound meaning. What could be finer, I thought, than to be a neurosurgeon?”

Dr. Marsh would become one of Britain’s foremost neurosurgeons, and in this unflinching book, “Do No Harm,” he gives us an extraordinarily intimate, compassionate and sometimes frightening understanding of his vocation. . . . .