Life is complex. And the older we get, the more complicated it can seem. Questions we thought we dealt with as teens-Who am I? What am I doing?-can crop up again in our thirties, forties, and even fifties. So how do we move past them this time?

Stephen D. Miller, MD, tackles these questions in his book "What Do I Do Now?," which provides a cohesive blend of answers, both from major world religions and from modern psychology-namely, that there are seven life phases common to humankind. The advice held within these pages will sometimes be as simple as "be nice" or "control yourself," but...