Duke and I by Julia Quinn

By all accounts, Simon Basset is on the verge of proposing to his best friend's sister--the lovely and almost-on-the-shelf--Daphne Bridgerton. But the two of the know the truth--it's all an elaborate ruse to keep Simon free from marriage-minded society mothers. And as for Daphne, surely she will attract some worthy suitors now that it seems a duke has declared her desirable....