An improvement over the second book in a number of respects. To catch you up, the setting is this: Conrad Schwartz, an itinerant engineer, is accidentally transported back to twelfth century Poland, 10 years before he knows Attila is going to sack the country with his Mongol horde. He proceeds (with a little seed support of the anonymous time travel agency) to rise in status, position, wealth and prestige within the local nobility as he works hard to modernize the country's infrastructure in the hopes of preparing to fight off the Mongols.
The first two books established the setting and what his goal was - to prepare Poland to repulse the Mongol invasion - but they moved at a very slow pace, often going into great detail describing the engineering challenges of recreating 20th century technologies without the industrial and machining infrastructures as well as the solutions to those challenges. After two books, only two years have passed, which led me to anticipate gritting my teeth for another eight volumes. But in The Radiant Warrior, Frankowski finally picks up the pace a bit: three or four years pass during the course of this one book. For most of that time, Conrad works to get the war production economy in motion. This involves so many tasks over such a long period that most of the tasks we are thankfully spared from an in-depth presentation, by the introduction of some Khazakh refugee artisans who are able, eventually, to train teams of people to do the necessary tasks that either Conrad or Frankowski doesn't know how to do and so are necessarily glossed over in the narrative. But Conrad is very busy in that time: first he creates a new base 12 numbering system and a system of standard measures based on the new numbers (Frankowski doesn't short-change us here; he's actually drawn up detailed charts&graphs of all the measurements - with conversion tables! - which he thoughtfully includes in ten pages of appendices after the narrative). He designs hot air balloons, creates the Boy Scouts organization, sets them to creating fixed-wing aircraft, creates the post office (expanded from the couriers serving his Playboy clubs), designs and builds a copper mine/refinery/forge, designs and builds a coal mine, a mechanism for mass-producing coal, and coal-burning stoves, designs and builds settlements to serve both of these installations as well as a separate community for the Khazar refugees, designs and constructs battle wagons with mounted swivel guns, and designs and implements a military base and the army training to go with it. In the midst of all that, he slays a bunch of Inquisitors who were about to burn a bevy of women accused of witchcraft, is given one of the Khazar's daughters (described as easily the most beautiful woman he's ever seen) as a slave after she dances the seven veils for him, marries off a bunch of his ladies-in-waiting, provokes his primary detractor into having a fatal heart attack, sees a priest's wife naked (she's the second most beautiful woman he's seen) after she slaughters her husband but before he essentially gifts her to his liege, learns a bit more about the genetically engineered intelligent dog-horse Anna as she starts to asexually reproduce, and is promoted to Baron. The fast pace, while necessary, is also somewhat frustrating, because the glossing-over of various details feels cheap at times.
The closest thing we get to an unified narrative is actually told from Piotr's point of view. Readers of earlier books will recall that Piotr is a skinny short kid who is a natural with numbers (apparently the base-12 numbering works well for him, too) and who has long been working as Conrad's accountant and money manager. Piotr is completely besotten with Krystyanna, one of Conrad's ladies in waiting who has been sleeping with him since she was a pre-teen. Throughout the book, Piotr tries to get the affection and approval of Krystyanna so they can be wed, but she repeatedly rebuffs his advances with apparent revulsion, and three of those times she immediately goes off and gets knocked up by Conrad.
But a lot of the book - and most of the second half - is concerned with how well the first class of the army training school is developing into trained, obedient killing machines. Piotr ends up joining them, and he - clearly the least of them - manages to survive basic training and ends up getting in a contrived fight with a fully-armored knight while he is stark naked. Of course he wins that fight, aptly demonstrating that Conrad's efforts are succeeding at revolutionizing combat. The "Radiant Warrior" of the title refers to the graduates of the first class, who have a graduation ceremony involving a vigil and the graduates going up on a hill above a foggy valley so that they can witness their glory.
This third book still suffers as the previous ones did from a lack of a sense of the protagonist - Conrad - as a real person, with any kind of emotional reality, but it has improved on The High-Tech Knight in its presentation of women...at least a very little bit. Years pass in this book, and by the end of The Radiant Warrior, the girls are now all fully-grown women of fifteen, sixteen, maybe even seventeen whole years of age. Conrad marries them off, one by one, to the men who become his vassal knights, and - here's the improvement - no mention is made of Conrad shipping in new pre-teens to replace the older girls in his household. For me, that was a significant change and made the whole book a lot more bearable. You still have to cross your eyes and hold your nose to stomach the idea that this man never had any qualms about sex with girls who would be severely underage in his 20th century origin. It's apparently easier to revolutionize the entire infrastructures of science and industry than to improve the social and physical conditions of women.
The book does an accomplished job of imparting urgency to deal with the Mongol invasion: both that Conrad is mentioning it daily, and that, with the Khazars, we have eye-witness accounts of the implacable merciless savagery of the Mongols. Those things go a long way to help us deal with the breakneck pace of the book: "Yeah, yeah, you've had it rough. Tell me again about how one Mongol lined everyone up in a queue, and then proceeded to go down the line, beheading each in turn, with everyone so terrified of him they couldn't act to defend themselves." The Mongols are bad news, indeed, so we're begging Conrad to get on with it and complete his preparation work so that he can defeat the Mongols when they invade.
There comes a time in every good role-playing game campaign where the game is no longer about character promotion - further "leveling" your character so that you access better and cooler powers - but is instead about building: building a fortress, founding a village, creating an empire, developing new trade routes, designing a new spaceships, and so forth. The part of me that finds that kind of game really compelling is the part that really enjoys these books, because they're all about that kind of creation. They're often fun, and mostly quick reads.