The Design of Everyday Things, by Donald Norman

The Visual Display of Quantitative Information (and its two sister books), by Edward Tufte

Zen and the Art of Motorcycle Maintenance, by Robert Pirsig

Concrete Mathematics, by Graham, Knuth and Patashnik

Any book about the theory of architecture, maybe Learning from Las Vegas by Robert Venturi

From everything2 entry on “Careers for Liberal Arts Majors”