Gödel, Escher, Bach by Douglas Hofstadter. Best described by its tagline: A Metaphorical Fugue on Minds and Machines in the Spirit of Lewis Carroll. Deeply profound ideas about the role of recursiveness, form and content.

Le Ton Beau de Marot by Douglas Hofstadter. Excellent exploration of the phenomenon of "creativity", the computational modelling of creativity, through the medium of translation.

Being There by Andy Clark. Excellent introduction to "Embodiment" -- the school of thought that includes body and environment as an integral part of cognitive systems.

Linked by Alberto-Laszlo Barabasi. About the discovery of free-scale networks and their pervasiveness in the natural and artificial worlds.

Death and Life of Great American Cities by Jane Jacobs. Classic of urban development.

Sexual Behaviour of the Human Male by Kinsey Pomeroy and Martin. A revolutionary (and controversial) attempt to objectively study the taboo.