I can't think of any books off hand, it'll vary depending on what kind of game you're making. Like the AI in F.E.A.R. will be different from Starcraft, which different than that in XCOM, etc.

And if you're scaling difficulty, then you're likely tuning how the AI behaves.

GDC, GDCVault, academic papers, and dev blogs//post mortems of similar games to what you're interested in will have a lot of good information.