A Picasso-studded penthouse owned by a fugitive financier provides the setup for a heist — and richly comic mayhem — in Donald E. Westlake’s latest caper. Unrepentantly boorish tycoon Preston Fareweather hides out from his vengefully grasping ex-wives in Club Med, leaving his Fifth Avenue aerie easy pickings for career thief John Dortmunder and his crew. One distraction: The gang’s meeting place, the O.J. Bar & Grill, has fallen prey to mobsters with loud shirts and louder manners. While the pace sometimes sags, Westlake compensates with uproarious portrayals of aging lowlifes overtaken by high tech (e.g., a car thief bedeviled by GPS tracking). In the end, Watch Your Back! delivers the goods as a breezy, satisfying lark.