I finished this yesterday, and once again, I found the beginning and end to be stronger than the middle, but I have a theory for why that may be. And it doesn't have to do with ERB; rather, it revolves around my work schedule. You see, I typically finish the previous novel as my work week is winding down, and immediately dive into the next one (I've already read 4 chapters of #15 last night). So my ability to maintain focus when reading before bed is hampered by a long day of working (when I'm in the middle of the story), and maybe I piddle out a few pages at a time, instead of whole chapters - sometimes reading several paragraphs more than once! ...sometimes reading several paragraphs more than once! ...and maybe I piddle out a few pages at a time, instead of whole chapters - sometimes reading several paragraphs more than once! [You get the idea.]
I mean, I was really struggling yesterday to complete the novel, even though I felt rested enough and really wanted to finish it. I had to stop, get up and move around, then come back again (repeatedly). I even did some of my vision fusion exercises in hopes my brain wasn't shutting itself down like it was two to three years ago, before I had vision therapy (for double vision). And I really did enjoy the story with one exception, which was both cool and a missed opportunity.
And had this book been published in the 1950s, instead of 1930-1931, it probably would've been called, "Tarzan versus the Reds". Oh, and the dynamic Neal Adams cover has nothing to do with the story...that I can recall.