Her Rogue to Ruin by Christy Carlyle

After losing her husband, Lady Portia Hastings has found her calling as a portrait artist, and she’s achieved enormous success among the noblemen and ladies of high society. As part of a widows club, she’s befriended other widows who are living boldly and seem to attract lovers with ease. Not so for Portia. Her natural shyness has always hampered her. Thus, her attraction to Phineas, Lord Pemberton, has gone unspoken, even when she’s summoned to his London townhouse month after month to paint the portraits of his mother and sisters.
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.