I'd like to say that the problems start with the first page. This would not be true, however--the problems start with the cover. Consider the image: a sprawled, underclothed woman idly and dangerously fingers a revolver shoved crotchwise into a man's fur-bikini waistband. The man is mustering all the dignity he can, despite his squatting position and the fact that he looks vaguely embarrassed about the tasseled hat somebody forced him to wear. Meanwhile, a toddler thumbing a flint knife gives the evil eye to the viewer.
And then you open the book.
The first twenty-odd pages retells and expands the last five or so pages from the previous of the series. In doing so it puts the lie to a statement about how a pair of antagonists "pass from the knowledge of men"...by continuing to talk about them for at least ten more pages. All of which is completely contradictory because the first person narrator has no way of learning their story before they disappear for good.
It's one of those Lin Carter books where the characters wander aimlessly and nothing interesting happens, despite the many battles and animal attacks and whatnot. The author seems compelled to tie up every loose end, and in doing so lacks any interesting set pieces or cohesive vision of what the book should be about (as evidenced by the vague and lackluster back cover copy). I didn't like it when Edgar Rice Burroughs wrote this sort of running-about jungle adventure, and I certainly don't enjoy it here.
It finally gets somewhere when they meet the lost World War Two German soldiers, but there's no interesting Secret Underworld Nazi Base set piece (or U-boat, to borrow interestingly from The Land That Time Forgot). Their presence is as much an afterthought as anything else in this novel.