"Doom of the Green Planet" is a 1968 science-fiction novel, written by Emil Petaja and published as half of an Ace Double. It is the sequel to Lord of the Green Planet.
Since I had not read the original book, I was dropped into this storia in medias res, and had to piece out a detailed backstory. Diarmid O'Dowd was a space explorer who found a planet that, millennia prior, had been conquered by a man from earth who had settled it with people who he kept under primitive conditions, with the two groups, "Nords" and "Islanders" representing Vikings and Celts, respectively. They were kept in medieval stasis, until O'Dowd liberated them and tried to change the state of eternal war and stasis. The premise of this book, at least from what it says on the cover, is that another space explorer, with selfish motives, wants to take over the planet for his own. But there are two other plots going on in this book: the Nords and Islanders, not quite ready to adjust to a peaceful world, are about to go to war again, and the deposed god-ruler of the planet is also not quite as gone as O'Dowd thought he was. The book handles these three plots in less than 130 pages, although the editor's knife is apparent in places. Although the book's premise could be seen as a type of gimmick, with ancient Ireland mixed with spaceships, this book managed to go beyond being just sword and sorcery to addressing some larger questions about society and its progress.
It is a compliment to a work to say that it leaves the reader wanting more. And this book certainly did that. I feel that this book could have easily been expanded to a 300 to 500 page range. It mentions concepts in passing that certainly could use expansion---such as sentient bacteria that perform "magic" by adjusting their shape over what to them is thousands of subjective years--- but are just mentioned for a page of background. Petaja had an expansive imagination, and this book left me wondering about his wider world.
Although Petaja is not widely read today, at the time this book was published, he had been writing for a long time, first publishing in the early 1930s, while still a teenager. He was apparently well-known inside of the science-fiction community, and so this story would have been the draw for the Ace Double. As is often the case, it was paired by the first novel of an unknown writer, in this case something called Star Quest by someone named Dean Koontz.