Like the Civilopedia in the Civ games.

It could have information and history about:
-The factions (advantages, disadvantages, units, great leaders, tactics that an AI using this faction might use, history)
-Provinces (location, special features, history)
-Units (stats, what it is good at, what it is bad at, history)
-Weapons (history)
-Tactics (that the AI will use, how to counter them, history)
-Battles
-Wars
-Game concepts
-People
-Events

And more maybe some more.