Poul Anderson's _The High Crusade_, good book for the "human conquest of the stars" theme.

In which a Medieval English Baron establishes a holy galactic empire (with themes of righteous rebellion, but it is against xenos, and so less antinomian than when human peasants pretend to self-determination).

Poul Anderson is good generally. He worked broadly across science fiction and fantasy.

- For hard science fiction, I would recommend his Tau Zero.
- For Nordic fantasy, I would recommend his The Broken Sword.

The High Crusade is somewhere in the middle.