What it says in the title. The manual is free for download on the Kalypso website, but it would be nice to have it included in the GOG release.

Ah i see it now, didn't know that there was an extra manual for the Expansion of this game.
https://download.kalypsomedia.com/manuals/

It's named differently then wiki shows it but guess the one to look for is called IR: Reign of Augustus

And yes it's missing in the game folder or GOG games library.
Don't think they will fix this soon if somebody doesn't contact them directly via support something.