”My mother enjoyed claiming direct descent from Genghis Khan,” writes Gray, and the comparison is apt. Tatiana du Plessix Liberman and her second husband (Gray’s stepfather), Alexander Liberman — shrewd, charming Russian émigrés — were ruthless in their conquest of Manhattan society in the 1940s and ’50s. She was the famed milliner (”Tatiana of Saks”) and he the brilliant Condé Nast editor; both were more dedicated to their opulent lifestyle and pedigreed social circle — Dietrich, Dior, Horst — than to parenthood, often sending Francine to live with friends. After Tatiana died at age 84, Liberman married her nurse. It was only after his 2000 death that Gray began writing Them, a task that, she says, gave her ”retrospective clarity.” It gives the rest of us an absorbing, elegant portrait of two unforgettable people.