Marlene Targ Brill 
Rough-Riding Adventure of Bronco Charlie, Pony Express Rider [PDF ebook] 

On a ranch in California . . . a boy dreams of becoming a pony express rider. California, early 1860s. Julius loves horses so much that he leaves his job as a sailor to work on a ranch. He is so good at taming horses that the other men at the ranch call him Bronco Charlie. He wants to become a pony express rider, but everyone tells him he s too young. When a pony express horse gallops into town without its rider, Charlie volunteers to take his place. Can Charlie deliver the mail and prove his worth as a rider?