From a Spiritual But Not Religious (SBNR) perspective I've enjoyed and benefitted from:
"What is the gospel?" by Greg Gilbert
"The Explicit gospel" by Matt Chandler
"The King Jesus gospel" by Scot McKnight
"The Lost Message of Jesus" by Steve Chalke
"The Mystery of Christ" (and why we don't get it) by Robert Farrar Capon
"Pagan Christianity" by Frank Viola and George Barna
"Christless Christianity" by Michael Horton
"Pharisectomy" by Peter Haas (on "How to Joyfully Remove Your Inner Pharisee and Other Religiously Transmitted Diseases")
"Christianity is Not Religion" by James Fowler
"Soul Rape" by Shean Smith