The Gospel-Centered Life by Will Walker and Bob Thune (Church Planters with Acts 29 Network)
See email from Barbara dated October 23, 2016 correcting timeline errors.
The Gospel-Centered Life was published by NGP with Serge in 2011 and has sold many thousands of copies, this small group resource is based on the ideas of Sonship and made into an easy to use resource for the local church.
Written by Will Walker and Bob Thune, both church planters with Acts 29 Network. They would be great people to interview because they are the next generation down from the first generation of leaders impacted by Sonship.
Also Daniel Montgomery from the Sojourn Network of churches would be another guy.