In her beautiful, suspenseful and timely new novel "American Dirt,’’ Jeanine Cummins succeeds in taking migration — one of the central issues of our time — and bringing it down to human size. The story of one Mexican mother and her young son reminds us that migrants at the U.S.-Mexico border (and everywhere) are individuals driven by their own unique and desperate stories. Cummins tells of Lydia, a bookstore owner in Acapulco, and her smart, sens…