Opening Repertoire: Nimzo and Bogo Indian: http://www.amazon.com/Opening-Repertoire-Nimzo-Bogo-Indian/dp/1781941092
Christof’s channel: https://www.youtube.com/user/Chessexplained
Christof’s 4.a3 Nimzo-Indian game vs. GM Desireless: https://www.youtube.com/watch?v=tACu3p0bE00