In “Ordinary Life,” Mavis McPherson locks herself in the bathroom for a week, shutting out her husband and the realities of their life together, and, no, she isn’t contemplating a divorce. She just needs some time to think, to take stock of her life, and to arrive, finally, at a surprising conclusion. In “White Dwarf” and “Martin’s Letter to Nan,” the secrets of a marriage are revealed with the sensitivity and “brilliant insights about the human condition” (Detroit Free Press) that have become a trademark of Berg’s writing.