They're Off! : The Story of the Pony Express;

In 1860, with North and South about to be divided by war, East and West were united through an extraordinary venture -- the Pony Express. Over the course of ten days in April, eighty riders and five hundred horses delivered mail between California and Missouri -- a mission that took three weeks by stagecoach.
Although it existed for only a year and a half, the Pony Express remains a legendary chapter in American history and a symbol of the bold, adventurous character of the Old West. The spirit of the brave riders and their ponies is captured perfectly in Cheryl Harness's lively, brightly illustrated, and information-filled account.