Mack "Lost Gospel" is a very imaginative book. After conjecturing a "Q" source for the common material of Luke and Matthew, he then assumes it to be the earliest of Christian writings. This writing has a definable community behind it which he describes with unwavering confidence. He then conjectures that it has three layers to it, each of which neatly coincides with a stage in this communities life.
Interesting enough, the earliest stuff is supposedly social criticism, and the later material is apocalyptic. Mack's Jesus ends up sounding a lot like Mack, with very little evidence. But its very imaginative...