Richard Osman's plots tend to fly over my head, but Osman is so clearly enjoying himself, I just keep reading. Osman's special talent is to exaggerate certain qualities we all have--while still making his characters recognizably human. In Osman's new novel, "We Solve Murders," Rosie, a ninety-year-old novelist with an insatiable appetite for sex, seduces various handymen while defending her own life against an assassin (and while, also, investigating a mystery). Rosie's sole moment of weakness seems to occur at an airport bookstore; if you watch closely, you might spot her hiding a Lee Child novel behind a title of her own. Elsewhere, Bonnie, a young woman, dreams of a career as "an influencer." She sees an opportunity: She could sit poolside, in swimwear, while advertising the virtues of "organic paint." ("I hadn't even known that paint could be inorganic ...") Bonnie rehearses her lines, in private: "Color brings happi...