The desk in Robert Caro's office has a rounded notch, a clean little half circle that lets him snug his wooden chair into his custom-made workstation. Instead of legs, the top rests on a pair of sawhorses. Shims raise the surface to where his elbows naturally rest when Caro's pen rolls across the white legal pads on which he writes the first drafts of his epic biographies.The height was calibrated by President John F. Kennedy's personal physician, Janet G. Travell, M.D.