Proposing to a Duke — Claudia Stone

At the age of five and twenty, confirmed spinster Isabella Peregrine is given an unexpected ultimatum by her new step-mother: find a husband or I'll find one for you.
Thoroughly in a tizz, Isabella proposes a marriage of convenience to the nearest male to her - who just so happens to be the brooding, intimidating Duke of Blackmore. When he rebuffs her clumsy advances, Isabella leaves for London, determined to find the man of her dreams and soothe her bruised ego.

Michael Linfield, Duke of Blackmore is a legendary, fearsome, war hero; famous for reducing debutantes and their mothers to floods of tears with just one glance. The brooding, giant of a man is hiding a secret however - a debilitating stammer that has haunted him since childhood. When the delightful Miss Peregrine proposes a marriage of convenience to him - with no benefits - Michael is so startled that his stammer reappears and all he can manage to reply is "N-n-no".
When news of Isabella's success in London soon reaches his ears, Michael is determined to go to town and win her as his own, if only she'll give him a second chance at love.

This sweet and clean Regency Romance is a stand alone novel with a guaranteed HEA.