Dallying with the Diamond - Andrea K Stein

Leonidas Croft Atherton, former cavalry officer turned society artist, has ferocious carnal appetites and little use for the Society that shunned him after his father, on his deathbed, pronounced him another man's son. His revenge on those high-in-the-instep members of the ton involves painting lascivious portraits of peers' mistresses and doing even more scandalous things with their wives.
When the shared journal in which he and his friends have recorded their sexual exploits goes missing, Leo will stop at nothing to prevent revealing the identities of their paramours. In his experience, angry husbands tend to shoot first, and ask questions as they reload.
Lady Honoria Eveleigh has been deemed the Season's Incomparable and is
all but engaged to the ton's most sought-after bachelor. Her almost
betrothed is heir to the wealthiest dukedom in England, next to her
father's, that is. He's handsome, kind, witty, but too terrified to even
try to kiss her. She started her secret collection of naughty books out
of curiosity. Now she fears reading about erotic desires that match her
own is all she may ever do.
Discovering pages from their journal have been sold to a duke's daughter
is bad enough, but now Leo must venture back into the Society that
abandoned him. Worse, said duke's daughter, Lady Honoria, blackmails him for the pages' return and tempts him to meet her licentious demands.
When a man who looks like sin incarnate is desperate for her help,
Honoria cannot resist the desire to see if what she's read about him is
true. No harm in a lady indulging her curiosity just once, is there?