An Improper Duke by Amy Rose Bennett

Having an affair with a heart-stoppingly handsome, much younger duke is all kinds of improper…but when he’s also your stepson’s best friend, it’s positively scandalous.
Bookish Celia Wyndham, the widowed Lady Ashdown, has been half in love—and shockingly in lust—with her stepson’s best friend, Hugo Dehavilland, the very eligible Duke of Tremayne for the longest time. When Hugo attends a week-long house party at Ashdown Hall—and then accidentally discovers he’s the object of the viscountess’s erotic fantasies—he suggests they embark on a passionate affair…a wicked proposition that Celia simply cannot refuse.