"Tarzan and the Ant Men" (Tarzan #10) by Edgar Rice Burroughs

No one had ever crossed the Great Thorn Forest until Tarzan of the Apes crashed his plane behind it. Within lay a beautiful country. But in it lived the Alali, strange stone-age giants whose women regarded all men as less than slaves. And beyond the Alali lay the country of the Ant-Men — little people only eighteen inches tall. There in Trohanadalmakus Tarzan was an honored guest—until he was captured by the warriors of Veltopismakus in one of the ant-men’s many wars. They had plans for the ape-man. They shrunk Tarzan to their size and set him to work as a quarry slave. Tarzan’s only hope is to make a daring escape. But what can an eight-inch-tall man do?