Chloe and the Devil by Carole Mortimer

Chloe, and five of her friends have formed an alliance, one in which they have agreed none of them will ever marry one of the arrogant and far-too-full-of-themselves gentlemen of the ton.

One of those ladies has already broken that alliance by falling in love and accepting a marriage proposal.
Chloe loved someone once, but she loved unwisely, and she now has no intention of ever succumbing to that devastating emotion again.
But when a journal is stolen from her bedchamber, some of its content scandalous, Chloe can think of only one person to ask to assist her in having that journal returned to her.
The handsome and icily imposing Lucien Lyons, the Duke of Hellsmere, is a close friend of her father’s. But Chloe knows he also secretly carries out work for the Crown. If anyone can retrieve the incriminating journal, Hellsmere can. Hopefully before its contents become public and Chloe is embroiled in a scandal she will never socially recover from.
Lucien has never been able to say no to Lady Chloe Lord, and he has no intention of failing her now when she is asking for his help.
Even if it will take every ounce of his self-control to resist taking her in his arms and making love to her, as he has long
ached to do.