2017/67: The Ruin of a Rake -- Cat Sebastian

... to combine scientific pursuits with actual orgies struck Julian as excessive in all directions.[loc. 63]

Julian Medlock is a shipping heir, the epitome of a Regency gentleman, whose carefully-polished exterior that armours him against the world and hides a number of secrets. Lord Courtenay is a notorious rake, penniless despite his aristocratic name, who has become bored of being bad. Courtenay is widely acknowledged to be the inspiration behind lurid bestseller The Brigand Prince of Salerno -- a novel which Julian knows rather well.

Eleanor, Julian's sister, is unhappy despite her unladylike scientific pursuits: she has surrounded herself with what appears to be a circle of reprobates, Courtenay chief among them. Julian feels responsible for his sister, whose unsuccessful marriage he helped arrange. Summoned by his sister's butler to 'rescue' her from the perceived depravities ensuing from her friendship with Courtenay, he finds himself involved in a scheme to improve Courtenay's reputation -- ideally without wrecking his own. He is uncomfortably aware of Courtenay's good looks: now he begins to realise that he's misjudged the man.

Charming, funny, and notable for having a protagonist who is good at accounts. Julian is, perhaps, his own worst enemy: but he and Courtenay mellow one another's less admirable traits, and even manage to communicate effectively. I think this is my favourite so far of Cat Sebastian's novels: and it's the third in the linked trilogy which began with The Soldier's Scoundrel. I realised I'd missed out the middle volume, The Lawrence Browne Affair, so set out to remedy that omission.