The Christmas Foundling - Belles of Christmas: Frost Fair, #5

Lydia Blakeburn, Baroness Lynham, wants nothing more than to present her husband with an heir, but after five years of marriage, she has all but given up on doing so. What started as a love match has since begun to sour, yet there is nothing Lydia can do to free Miles from a marriage she is certain he regrets. The future they had envisioned together is slipping farther and farther away. Miles Blakeburn is at his wit's end.
Not only has he failed to produce an heir to the family's ancient title, his wife has withdrawn from him entirely. The one thing in the world she wants, he has thus far been unable to give her: a child. He is a failure in every regard. When they happen upon an abandoned baby at the Frost Fair, Miles simply can't resist the light he sees sparked in his wife's eyes as she holds the child. He agrees to take the foundling in until Christmastide is over, when they will find the babe a permanent home.
But, for a couple who's been yearning for a baby for years, the infant can't help but expose all the feelings Lydia and Miles have long been trying to bury, feelings that have the power to bring them back together-or estrange them forever.