Befriending Burgess by Iris Lim

She feeds him information to keep the title hunters at bay. He starts with wanting her help, and ends up wanting her heart.

A duke, even a reluctant one, is duty-bound to marry and to sire his own heir. When the savagery of the marriage mart overwhelms the quiet, scholarly Duke of Burgess, he turns to a friend to help him repel the worst of the bunch.
Newly emerged from mourning her father, the unassuming Rose Nottingham believes herself outside most eligible men’s notice. Instead, she lends her knowledge of the ton to helping her beleaguered friend Frederick St. John, the Duke of Burgess. But watching him court other, more eligible ladies turns increasingly harder the deeper her own feelings grow.