All of my children have gone through (and continue to go through) from time to time a cowboy phase. Their interest is usually stoked after watching a John Wayne movie with Daddy. They especially enjoy a rowdy game of Cowboys 'n Germans outside (their uncle told them it wasn't politically correct to play Cowboys 'n Indians).
Three cowboy/cowgirl-themed pictures books found their ways into our library tote this winter.
My favorite was Kid Sheriff and the Terrible Toads by Bob Shea, illustrated by Lane Smith. It is cute and smart. A small kid rides into an Old West town overrun by the Terrible Toad gang. He becomes Sheriff Ryan, and he uses his expansive knowledge about dinosaurs to round up the bad guys. This book is filled with fun cowboy language and great humor. The only problem I see somebody possibly having with it is that it uses the phrase, "Dang it." I'm not fond of my kids using this phrase, but it just fits this rough and tumbleweed town.
My girls picked out two books in a series: Every Cowgirl Needs a Horse and Every Cowgirl Needs Dancing Boots by Rebecca Janni with illustrations by Lynne Avril. Nellie Sue is a suburban girl who is a cowgirl at heart. Like Ladybug Girl, Nellie Sue has a little personal issue to overcome in each book, and she does so admirably. Not only that, her voice shines through in language alive with all sorts of fun cowboy metaphors and similes. The books are completely worth a read and would make great gifts for girls.
My advice is to round all of these books up at your local library for fun reading time.