''The God Delusion'' is a clarion call to the faithless, the waverers, and even firm religious believers, to follow the author into radical atheism not merely as a private conviction but as a public profession. Wouldn't humankind be better off without religion, he asks. John Cornwell's ''Darwin's Angel'' is not so much a combative repudiation of Dawkins' arguments as a playful conversation with them, posing alternative view-points, exposing lapses in logic and errors of fact, from the vantage point of a friendly Guardian Angel.