The Age of Spiritual Machines by Ray Kurzweil is an ideal non-fiction, but largely conjectural compliment to Calculating God by acclaimed Toronto science fiction author Robert J. Sawyer. Accessably written and logically based on foundations as diverse (or not!) as natural selection and microchip and nanotube development, Kurzweil's vision of our just-begun 21st century provides a fascinating explanation of how the human race might follow civilizations alluded to in Calculating God that uploaded themselves into vast computer systems. Indeed, Kurzweil prognosticates that the greatest moral debate in the second half of our new century will be how to differentiate between what is man and what is machine. For more on the author, see www.kurzweiltech.com.