The Caspak Series by Edgar Rice Burroughs

This book-collection file includes The Land that Time Forgot, 1918; The People that Time Forgot, 1918; and Out of Time's Abyss, 1918. According to Wikipedia: "Edgar Rice Burroughs (September 1, 1875 – March 19, 1950) was an American author, best known for his creation of the jungle hero Tarzan and the heroic John Carter, although he produced works in many genres."