Her Rogue to Ruin (Wanton Widows Club #3)
Christy Carlyle

Phineas Cross, Viscount Pemberton, has a list of worries that includes getting his four sisters married, keeping his mother from meddling in every aspect of his life, and winning the attention of the mistress he's desired for months. When that potential mistress sets him the challenge of gifting her with a nude portrait, there's only one person he can think to ask. Lady Portia Hastings is lovely and proper, and he feels like a cad asking her to do anything even slightly scandalous, but when she makes a scandalous request in return—that he give her lessons in seduction—he finds the prim widow impossible to resist.