The Land of Hidden Men by Edgar Rice Burroughs

Asia, vast continent of ancient civilizations and mysterious peoples, has many corners little known to the rest of the world. One such was the jungle-hidden heart of exotic Cambodia, where American explorer Gordon King stumbled upon a secret kingdom lost for a thousand years.
King faces danger, rescues a “jungle princess,” and braces the perils of the lost city of Pnom Dhek. Great Stuff! One of ERB’s less-known works, and a good one!