I am reading the novel Shogun.

The main character is a pilot, one who knows the secrets of how to navigate across the ocean (vital knowledge given the inability to tell latitude). The men who crew the ships have no ability to control where they go. Without a pilot they are hopelessly lost.

Each pilot keeps a diary called a "rutter" in which they record their travels and navigation notes. These are secret. Countries share information internally but keep it secret from others. And even internally much of the information is secret and known only to a few.

It has a very strong 40k flavor with the navigators and all of the secrecy.

It also has a lot on the conflicts within the christian church - and even within the catholic church.

It is undoubtedly the most 40k-ish flavor I've read in a non-40k book. Even moreso than most 40k books - which seems odd, but this novel is attempting to describe and explain the world, not just tell a story in it.