"Into the Darkness" is the first part of Harry Turtledove's six-volume reworking of the World War Two story set on a planet where technology is based on magic rather than machines.
Dragon riders replace aircraft, Behemoths replace tanks, East and West have been transposed, Eurasia has been moved to the Southern hemisphere so that Scandinavia becomes equatorial, and names and superficial national characteristics have all been changed. But this is real history, not alternative history. Again and again the terrible events of the book are based on real historical incidents.
Some of the changes to racial characteristics are impishly amusing, such as the fact that the people who correspond to the Finns live in an equatorial climate and look like Zulus, while the Saraha Desert becomes "the land of the Ice people," the Gyongyosian people who correspond to the Japanese are physically large, and the Kuusamans who correspond to Americans have epicanthic folds.
Other changes are rather more biting - the "Kaunians" who correspond to Jews are tall, blue-eyed, and blonde.
What Turtledove appears to be trying to do with this series is to study how different people responded to a time of great evil. Some people were sucked into taking part in that evil, some fought against it, others just tried to live through it. The changes to the names and characteristics of the participants seem to be intended to give the reader an opportunity to leave behind some of our emotional baggage about the holocaust so that we can try, not to justify the wrongs which people did in terrible times, but to understand how it could have happened.
All but two or three of the characters in the first few books books are fictional - Hitler is King Mezentio of Algarve, Stalin is King Swemmel of Unkerlant, and Marshal Rathar gradually morphs into Zhukov. This actually makes the story more exiting, as the characters are presented well enough that you care about them: we all know how World War II turned out but the readers has no such certainty about the fate of the fictional characters.
The six books of the series each corresponds very roughly indeed to about a year's real historical events. The first book, "Into the Darkness", mostly covers events corresponding to those between the start of the fighting when Hitler invaded Poland to the fall of France in 1940: the last few pages of the book are mostly filler taking the story up to set the scene for Operation Barbarossa, Hitler's attack on Russia in 1941, which is covered in the second book, "Darkness Descending."
The series is best read in the correct sequence. All the books of this series have the word "Darkness" in the title, but the publishers refer to it as the "Derlavi" series, this being the name given in the books for the great continent which corresponds to Eurasia. It is sometimes also described as the "World at War" sequence. The full set of six books in their correct order is:
"Into the Darkness"
"Darkness Descending"
"Through the Darkness"
"Rulers of the Darkness"
"Jaws of Darkness"
"Out of the Darkness".
Bottom line: the mood is as black as the titles indicate, but the series is a very exciting read.