Dave Shiflett is a knockabout writer. He did Trump's pre-presidential run book, probably a number of other ghostwriting episodes (like our Jay Homnick), a book defending Christianity from various spurious or somewhat spurious charges, and now one on the growth of conservative churches.
This last book is hardly necessary. Dean Kelley (of the liberal National Council of Churches, of all places) documented the trend decades ago. The reason for the loss of members to liberal churches has been commented upon repeatedly: there's no God there. The liberal churches simply proclaim a spirit of the age who is the same person as the people in the seats. Don't need no Savior. We're doin' just fine. "I'm okay. You're okay." Or something like that.
Nevertheless, Shiflett has done numerous interviews for the new book and it may prove interesting. He's done two pieces for NRO about it. Read the latest here.
I almost forgot, the title of the book is Exodus: Why Americans Are Fleeing Liberal Churches for Conservative Christianity.