The popularity of these novels lies in the cool and powerful Imperial Knights. They are these huge, imposing mechs with a rich lore behind them. People are drawn to their epic battles and the sense of honor they represent.
One great starting point is 'The Knight of the Blazing Sun'. It gives a really good introduction to the Imperial Knights in the Warhammer 40k universe, with an engaging story that shows their power and honor.
One of the highly regarded Imperial Knight novels is 'The Knight of the Black Rose'. It weaves a great story of honor and battles in the Imperial Knight universe.
One of the great 'Imperial Fists 30k' novels is 'The Horus Heresy: Pharos'. It delves deep into the lore of the Imperial Fists during that era. It shows their unwavering loyalty and their struggle in the face of the growing chaos.
The 'imperial knight freeblade novel' likely focuses on the adventures of an imperial knight who has become a freeblade. It might explore their battles, their code of honor, and how they operate outside the normal structure of the imperial knight orders.
There might be a Chapter Master as one of the main characters. He would be in charge of leading the Imperial Fists, making crucial decisions in battles and for the future of the Chapter.
One great 'Warhammer 40k Imperial Guard Fanfiction' is 'The Last Stand of the 13th Platoon'. It really shows the tenacity and sacrifice of the Imperial Guard. The way the characters are developed makes you feel like you're right there in the thick of the battle with them.
The central character is the imperial knight freeblade, of course. But there could be a whole cast of characters around them. There might be other knights who don't understand the freeblade's choice and see them as an outcast. There could be a group of imperial guardsmen who end up fighting alongside the freeblade. And on the enemy side, there may be powerful psykers or daemonic entities that the freeblade must battle against.
The main character is a delivery man who is at the heart of the story. There are also other characters like the people he meets during his deliveries, some of whom become his friends or enemies in this post - apocalyptic world.