In the very first sentence of “Conundrum,” the mesmerizing first-person account of Jan Morris’s metamorphosis from a man to a woman, the author, one of Britain’s most lyrical travel writers and a transgender pioneer, tells readers that she knew she’d been born into the wrong body when she was just 3 or 4 years old. “What triggered so bizarre a thought I have long forgotten,” she adds, “but the conviction was unfaltering from the start.”

Nicole Maines, the heroine of Amy Ellis Nutt’s “Becoming Nicole: The Transformation of an American Family,” evinces similar conviction about her gender identity, and at an even younger age. Shortly before turning 3, when Nicole was still known as Wyatt, he declared to his father, “I hate my penis.” All the while, the Maines household was running a control group of one: Wyatt had an identical twin brother named Jonas, whose sense of his own maleness was never in doubt. Once, when they were still small, Wyatt hit Jonas. Asked why, Wyatt replied, “Because he gets to be who he is and I don’t.”

Parents almost never get the children they expect. It’s part of what makes raising kids so glorious, this element of surprise; but it is also what makes raising them so hard, revealing our shameful vanities and previously undetected limitations. The Maineses adopted their boys, so they, perhaps, had fewer illusions about seeing the best of their genetic selves magically reappear in the next generation. But neither of them anticipated quarreling over whether their little boy should one day be allowed to go to school in a blouse.

Reading strictly for plot, “Becoming Nicole” is about a transgender girl who triumphed in a landmark discrimination case in 2014, successfully suing the Orono school district in Maine for barring her from using the girls’ bathroom. But the real movement in this book happens internally, in the back caverns of each family member’s heart and mind. Four ordinary and imperfect human beings had to reckon with an exceptional situation, and in so doing also became, in their own modest ways, exceptional.