I thought it would be kind of cool to have a running thread with a list of books that we've found helpful. Over the years, there have been many, but I thought I would list a couple here to get things started.
-- Lila, the follow up to Zen and the Art of Motorcycle Maintenance.
Author: Robert M. Pirsig
Paperback: 480 pages
Publisher: Bantam; Reprint edition (November 1, 1992)
ISBN: 0553299611
If you haven't read the first book, you can probably get away without it, but you really should read 'Zen' first. This book explores futher Pirsig's inquiry into the true nature of Quality.
-- The Physics of Immortality
Author: Frank Tipler
Paperback: 560 pages
Publisher: Anchor (September 18, 1997)
ISBN: 0385467990
I used this book for researching a novel I wrote, and it came in very hand. It's a very detailed description (read: lots of math) describing exactly, in theory, the physics of immortality would be. A bit technical, but a good read.
-- The Age of Spiritual Machines
Author: Ray Kurzweil
Paperback: 388 pages
Publisher: Penguin Putnam (January 1, 2000)
ISBN: 0140282025
If you work with technology, this is a must read. It's a bit disturbing, because Kurzweil's thesis is that we will create the machines that will render us obsolete, but it's very well written and explores some interesting facets to our consciousness.
I'll post more here, but this ought to get things started.