What Paul says is the Kilkenny order is incorrect.
#1 - The Rider of Lost Creek
#2 - The Mountain Valley War
#3 - Kilkenny

In the first #1, he meets Nita, In the #2, she follows him (it is in the book), and #3, Nita and Kilkenny have come together finally.

You can also throw in the short stories and novellas.

Short story "A Gun for Kilkenny" reads like an intro to Kilkenny with him only showing up at the end.

Short story "West of Dodge" is all Kilkenny but he is a loner at this point.

Novel "The Rider of Lost Creek" is all Kilkenny. He picks up Rusty Gates as a saddle partner. And, of course, he meets and falls in love with Nita Riordan.

Novel "The Mountain Valley War" has Kilkenny and Cain Brockman mend their fences. And, Kilkenny and Nita embrace their romance.

Novella "Monument Rock". I thought this was made into a novel but can't match it up. But, this ends with Kilkenny and Nita saying they are going to head for California.

Novel "Kilkenny" ends the Kilkenny saga with Kilkenny and Nita talking of getting married. Looks like they didn't make it or stay in California as this novel is set in Utal.