Caspak (Complete) by Edgar Rice Burroughs
Presented in the order in which they were published
The Caspak Series is a trilogy of books featuring the island of Caprona. (The island is also called Caspak by its native humanoid inhabitants – thus the name of the trilogy.) In the first novel, Caprona is described as a land mass near Antarctica and was first reported by the (fictitious) Italian explorer Caproni in 1721, the location of which was subsequently lost. The island is ringed by high cliffs, making it inaccessible to all but the most intrepid explorers, and has a tropical river teeming with primitive creatures that are elsewhere extinct.