To unlock the secret of a mysterious talking stone, Doc travels to an ancient utopia high in the mountains. A vicious army of vipers are hot on his heels, racing him to the city in the clouds. To get the secret, Doc's awesome talents are soon tested when he must protect the lives of his crew -- and the lives of everyone in the city!

Since this one had no comments or reviews, I decided to read it next and then provide my take on the story. The Speaking Stone starts out a little bit oddly, with Doc acting in a mysterious manner and not much going on except for a Doc associate (Long Tom) getting getting captured and subsequently disappearing. I was afraid that this was going to be one of the clinkers in the series but I persevered and was well rewarded. The trick in this tale is a mysterious stone which talks, that is, it repeats voices or other sounds. And it works, it really speaks. The thing comes from a strange, high altitude, utopian kind of society in South America. And as the story begins picking up steam, it turns out that there is a gang of bad guys who are after the source and the secret of this stone, thinking it will make them a lot of money. Meanwhile, Monk and Ham had been working in South America and had somehow gotten mixed up in this thing. As the story develops, it becomes clear that the two are in some kind of trouble. Doc rescues Long Tom and they set out to help their friends out of the jam they are in. Renny and Johnny are not in this story at all.
A quarter of the way through the story, it has gathered enough force make you want to keep reading. The ending is a little trite and the secret of the stone turn out to be a mish mash of scientific mumbo jumbo of dubious credibility. Nonetheless, being a latter Doc, he is more human, not the all powerful superhuman kind of hero that he is portrayed to be in the earlier stories. He still has plenty of intellect, courage and strength, however. The bad guys are fleshed out better than in the earlier stories and the descriptions of the high altitude scenery, cold and rarified air conditions are first rate. After a slow start, the tale rolls along nicely with a few suprise plot twists that make it enjoyable. There is plenty of action and at one point (of course) the associates think that Doc has been killed. You'd think they would know better by now. We do, and we're just the fans of the stories.
Anyway, I give this one a 75% rating, not the best but far from the worst and well worth reading despite a slightly weak ending.