Fifteen years of negotiation culminated in 2010, at the Review Conference in Kampala, with amendments to the Rome Statute of the International Criminal Court and "Understandings" as to their meaning. Resolution RC/Res.6*
There is a specific subject search to use in SOLO Aggression (International law).
But remember this topic will probably be included as chapters in books looking at international criminal law more generally.
Aggression under the Rome Statute: An Introduction by 
Crime of Aggression : a Commentary by 
Travaux Prèparatoires of the Crime of Aggression by 
The Crime of Aggression under the Rome Statute of the International Criminal Court by