The Express Riders(1982)
(Book 28 in the Making of America series)
A novel by Lee Davis Willoughby (William L DeAndrea, Richard Deming and Richard Laymon)
"1860: The Pony Express ran on the sweat and blood of headstrong young trailblazers. Fighting hostile Indians and greedy white man alike, the Express Riders forged a perilous trail as they carried the U.S. Mail through the untamed wilderness from Missouri to California."
Used availability for Richard Laymon's The Express Riders
September 1982 : USA Paperback