The book's about Ernest Hemingway and the third woman to make the grievous error of marrying him, Martha Gellhorn. It follows Hemingway around the time he and Pauline moved to Key West, and Gellhorn from much earlier in her life, as she heads out to try and become first a journalist, and then a writer. She travels a lot, meets various men, most of whom seem to fall madly in love with her, while she can only stand to be around them for brief stretches of time. But when she does want to be around them, she's as passionate about it as they are.
She eventually meets Hemingway on a family trip to Key West, they fall for each other, he starts seeing her on the (not very) sly. They travel to Spain to cover the Spanish Civil War together. Eventually they get married. Things go south from there. Martha grows more comfortable as writer, starts to experience success on her own, where she isn't quite so chained to Hemingway's legacy, which irks him. The more he tries to control, the more she pulls away, the more resentful he gets, the more she sees all the parts of his personality she doesn't like so much. Like the constant drinking and partying with his vast array of hangers-on and buddies.
In the broad strokes, there's nothing here I didn't already know. Martha and Ernest were terribly suited for a long-term relationship together, in large part because Martha was not going to spend her life as "Mrs. Hemingway". Ernest is a jealous jerk who destroys every friendship he has. Martha, while a passionate journalist, was not above ignoring facts that went against the cause she favored. But some of the details were new, such as Martha's efforts to get into mainland Europe and cover the Allied invasion in 1944 (despite Ernest getting her fired from Collier's). Or their travels in China.
Tuccille effectively highlights the differences in their perspectives and personalities through their reactions. Such as Martha not doing so well with the hygienic conditions in China, while Hemingway seemed unperturbed - until his booze ran out, naturally. Or Martha's tendency, pointed out to her by Hemingway at one point, to think everyone should think and react to everything just as she did. Hemingway, of course, was fully aware other people had different opinions from him. He just didn't particularly care what they were (he didn't say that, that's just my assessment of him).
'Ernest, undoubtedly, was suffering from guilt pangs over his treatment of Pauline. Years earlier he had blamed Pauline for breaking up his marriage to Hadley, and now he was finding reasons to blame Martha for his all-but-doomed second marriage. Someone had to be the scapegoat, as long as it wasn't Ernest.'