“THIS BOOK IS A STUDY OF PASSION,” writes Stephen Galloway in Truly, Madly: Vivien Leigh, Laurence Olivier, and the Romance of the Century, out from Grand Central. It’s true that there’s passion in spades—not to mention a cameo by Marilyn Monroe, glitzy dinners, and cinematic visits to the sets and stages of Antony and Cleopatra and Gone With the Wind. But Leigh and Olivier’s affair, marriage, and eventual divorce is a complicated saga, inflected by Leigh’s mental illness, which was widely dismissed and misunderstood. (“Nonsense,” Noël Coward told Olivier, when the actor raised concerns.) While past depictions render Leigh difficult and Olivier long-suffering, Galloway makes clear their symbiosis—Leigh “did her best to care for Larry” too, joining him as he filmed Henry V “to assist and advise and encourage and…