Rake in Disguise. Wicked Widows' League, #35

The most dishonorable thing Dr. Orlando Valentine could do was fall in love with a married woman. But when he witnesses Lady Blythe Clay being auctioned by her husband, he could not stand by and let another soldier claim her. Though she may be his, Blythe was still married in the eyes of the church, and he could not be a participant in adultery. Lady Blythe Clay had suffered the ultimate humiliation when her husband decided to sell her to one of his fellow soldiers.
Thankfully it was Dr. Orlando Valentine, a captain and physician in Wellington's army, who won her. Unfortunately, Orlando treated her more as friend and never a wife, so when she returns to England a widow, Blythe vows to never speak of what happened.  She had also hoped to forget Orlando, but when she comes face to face with him three years later, Blythe realizes that her heart never healed and the forbidden desire never died and she knows exactly what to do.