Andy Briggs 
Savage Lands [PDF ebook] 

The treacherous Lord Greystoke arrives at the logger’scamp in the Congo, and will stop at nothing to find and assassinate his long-lost cousin, Tarzan. Robbie and Jane set out through the jungle to warn Tarzan, and find themselves trapped in the sprawling ancient city of Opar, where the insane Queen La rules an army of man-eating apes. With a volcano threatening to explode, Tarzan, Robbie, and Jane must form an unlikely alliance to save themselves. Not everyone will make it out alive.