I'm reimplementing an old 4X space-rts game which has diplomacy options. The original was based on a relation scoring system (0..100) and a set of negotiation options (improve relations, alliance, declare war, etc.) The AI player usually had 3 options: yes, maybe and no; each adding or removing some amount to the relation score.

• How should the AI chose between the options?
• How does the diplomacy work in other games and how are they imlemented?
• Any good books/articles on the subject?

(Googling the term diplomacy yields the game Diplomacy, which is unhelpful.)