Biography is the most democratic of the literary forms, in the sense that it assumes its subject to be representative — that the specific life of a particular person contains within it something universally instructive.
The appeal to midlife readers is not difficult to understand. By fifty, most women have accumulated enough experience to recognize the patterns of a life — to see, in the choices another person made, the choices that faced them.
The lives of women make for particularly compelling biography. The biography of Mary Wollstonecraft, or George Eliot, or Simone de Beauvoir — these are stories of what becomes possible for women who refused to accept the limitations assigned to them.
The biographies that stay with us are the ones that make us feel less alone in our own complications — that show us, through a specific life lived with honesty and difficulty, that the struggle is universal and the resilience is possible.
Begin with a life that interests you genuinely — not the life you feel you ought to be curious about, but the one that actually makes you want to know more. The curiosity is the guide. Follow it.