Pride of the West
Paramount 1938
George Hayes-Windy
Russell Hayden-Lucky Jenkins.

Life magazine June 12, 1950
Article on the Hoppy phenomenon.

(Left) Hopalong Cassidy's Private War, Grosset & Dunlap 1950 reprint of Bar-20 Days (McClurg, 1911)
(Right) Hopalong Cassidy a reprint of McClurg, 1910

Hopalong Cassidy Takes the Cards, Doubleday, 1937.

Hopalong Cassidy Returns, Doubleday, 1924.

The Coming of Cassidy, McClurg, 1913.
Reprinted as The Coming of Hopalong Cassidy by Grosset & Dunlap in 1950.

Lost Canyon
United Artists, 1942
Andy Clyde-California
Jay Kirby-Johnny Travers

Hopalong Cassidy Serves a Writ, Doubleday, 1941
Last Mulford novel.
Filmed in 1942 as Hoppy Serves a Writ.

William Boyd as Hopalong Cassidy.

William Boyd as Hopalong Cassidy.

Hopalong Cassidy and the Bar-20 Cowboys.
A Little Golden Book for kids.

Call of the Prairie
Paramount, 1936
Jimmy Elllison-Johnny Nelson
Chill Wills and His Avalon Boys

Heart of Arizona
Paramount, 1938
George Hayes-Windy
Russell Hayden-Lucky

Hopalong Cassidy's Western Magazine,
Fall 1950
Lead story by Louis L'Amour writing as Tex Burns.

Twilight on the Trail
Paramount, 1941
Andy Clyde-California
Brad King-Johnny Nelson

Hopalong Cassidy's Western Magazine,
Winter, 1951
Lead story by Louis L'Amour writing as Tex Burns.

The full text of the first Hopalong Cassidy novel, Bar 20, can be found here.
A review of the Francis Nevins biography of Clarence Mulford can be found here.