Most of the best source material for case studies and best practices comes direct from firms who are implementing net promoter programs. As you might imagine, we get quite a lot of material from the conferences and net promoter.com
But for an executive audience, one of the best events has proven to be the Bain Loyalty Forum - a roundtable of senior execs with a common NPS mission. My notes on Verizon and Logitech both came from these meetings, but these events provide a pretty in-depth view into best practice as well as building a senior network to draw on. Several folk had requested information about this, so your best bet is to contact Stu Berman at stu.berman@bain.com for information.
See you there.