The Rancher's Runaway Bride (Men of Glory #1)
Judith Bowen

Then, one fine September day, he meets Nina O'Shea O'Sullivan on the road to Glory. She seems to be attracted to him, and he's sure attracted to her. They begin exchanging letters ... and the following spring, their long-distance romance leads to marriage.
The rancher brings his new bride home to Glory, but there's something he doesn't know about Nina, something her letters didn't tell him. She's not exactly who he thinks she is!
That summer, when she leaves, there's another thing she doesn't tell him: she's pregnant.