Perhaps the most poignant moment in “King Lear” occurs when Lear, on the cusp of losing his mind, cries out, “O, let me not be mad, not mad, sweet heaven; keep me in temper; I would not be mad!” Echoes of Lear’s terror resound through Matthew Thomas’s stunning first novel, “We Are Not Ourselves,” as one of the pivotal characters succumbs to the ravages of early-onset Alzheimer’s.