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For a harder read, you will need to go back to Richard Feynman鈥檚 Caltech
Lectures on Gravitation. You will need a sound understanding of theoretical
physics to get the most from this collection, but as Kip Thorne and colleague
say in their introduction to the new edition: 鈥淭here is much . . . that will be
valued by physicists, students, historians, and admirers of Feynman. Moreover,
the lectures are fun. Many passages offer a glimpse of a great mind approaching
deep and challenging questions from an original perspective.鈥� Published by
Penguin, 拢18.99, ISBN 0140284508.