Hopalong Cassidy's Rustler Round-Up; Or, Bar-20 is a Western novel by Clarence Edward Mulford. It tells the story of Hopalong Cassidy, a cowboy who helps to round up a gang of rustlers who are stealing cattle from the Bar-20 ranch. The book is set in the American West in the late 19th century. It features a cast of memorable characters, including Hopalong Cassidy, a skilled cowboy and a capable gunfighter; Red Connors, Hopalong's best friend; and the rustlers, a group of ruthless criminals who will stop at nothing to get what they want. The book is full of action and adventure. It features gunfights, cattle drives, and chases across the desert. It also explores themes of friendship, loyalty, and justice. Hopalong Cassidy's Rustler Round-Up; Or, Bar-20 is a classic Western novel that has been enjoyed by readers for generations. It is a well-written and exciting story that features a strong cast of characters and a timeless setting.