Many times, you need to prove to your customer and business people the value of adopting Agile mindset, methods and practices. The following resource are very valuable in this regard:
1) The Business Value of Agile Software Methods Published by J. Ross Publishing, 2009
This book has useful info on business value and return on investment of Agile methods and practices.
2) There is a report named "2015 State of Agile Development" from CEB (Corporate Executive Board) Applications Leadership Council.