Lucky Penny (Coulter Historicals, 4) by Catherine Anderson

To support her orphaned niece, impoverished Brianna O'Keefe accepts work with a Colorado rancher. To guard herself from the unwanted attention, she resorts to a harmless little lie: that she's married to a Denver gold miner named David Paxton. When her boss forces her to write her "husband," hoping he'll take Brianna off his hands, she can only pray that there is no real David Paxton who can expose her.

When Colorado marshal David Paxton gets a letter from a stranger claiming to be his wife and pleading with him to come for her and his daughter, he dutifully sets out to find this woman and the child he may have sired. What stuns Brianna is the David is convinced the young girl is, in fact, his. As David and Brianna's wary attraction blossoms into deeper desire, David warms to the idea of a ready-made family. But can his dream survive Brianna's lingering distrust—and his own secrets?