The book is Shopping for God: How Christianity Went From in Your Heart to in Your Face by James Twitchell. You can read reviews of it here and here. There is an interview with the author here. About a year ago, I had an entry about a similar book. You can read that entry here.
One of Twitchell's main points is that competition in the religion market is fueling the rise in mega churches. If a church wants more members, it has to offer a better deal and more benefits than other churches