The Lying Room by Nicci French

It should have been just a mid-life fling. A guilty indiscretion that Neve Connolly could have weathered. An escape from twenty years of routine marriage to her overworked husband, and from her increasingly distant children. But when Neve pays a morning-after visit to her lover, Saul, and finds him brutally murdered, their pied-à-terre still heady with her perfume, all the lies she has so painstakingly stitched together threaten to unravel.