Philosophy
The Story of Philosophy: The Lives and Opinions of the World's Greatest Philosophers, by Will Durant. Written in the 30s, it covers some philosophers who would not be considered influential today, and doesn't do justice to the 20th century. But it's a relatively easy read, and clarified some guys I either didn't read or didn't appreciate in my freshman great books seminar.
Walden and Civil Disobedience, by Henry David Thoreau. Ah, the wisdom. Likewise The Essential Gandhi, by Mahatma Gandhi. Haven't read all of it or even as much as I should, but his autobiography of his early years and experiments with truth are the story of a great soul.
The Mind's I: Fantasies and Reflections on Self & Soul by Douglas R. Hofstadter. Various attempts to reconcile the mind-body dichotomy and build a science of cognition.
Human behavior
The Naked Ape: A Zoologist's Study of the Human Animal, by Desmond Morris. If you ask people why they act the way they do, they will probably talk about rational, moral, idealistic justifications... but if you were a primatologist, what would you say about why humans act the way they do? Morris, a zoologist familiar with great apes, describes how human behavior may owe much to ape biology and society as well as to big brains and rational and spiritual pretensions.
Invitation to Sociology: A Humanistic Perspective by Peter L. Berger. Humans don't really have the capacity to create new behaviors on the fly when faced with new situations, or to respond in real time to new behaviors in others. Instead people internalize roles and scripts that they act out in daily life with small variations, interpret others' actions in light of performances they have seen before, and collectively build shared narratives in a 'social construction of reality'.
How We Know What Isn't So: The Fallibility of Human Reason in Everyday Life by Thomas Gilovich. A grand tour of the fallibility of homo economicus, and reality torturing heuristic shortcuts that lead us astray.
Influence: The Psychology of Persuasion, by Robert B. Cialdini. How markets, salesmen, lawyers, small children and other devious hucksters take advantage of our decisionmaking shortcuts for fun and profit.
Happier: Learn the Secrets to Daily Joy and Lasting Fulfillment by Tal Ben-Shahar. Came upon these after reading about his lectures, the most popular at Harvard. A lot of very sound advice to have a more positive outlook. I wish this influenced me more LOL. But applying it is no mean feat. Dan Gilbert also has Stumbling on Happiness, but haven't read that, and Gretchen Rubin, niece of Robert, has a bestseller - happiness is turning into a self-help fad and industry, but it seems empirically based and common sensical.
Economics
The Road to Serfdom, by F. A. Hayek. A powerful case against government overreaching and socialism. Focuses on the long run dynamism, information efficiency and evolutionary adaptability of markets which is lost when governments attempt to tame their shorter term inequities. Capitalism and Freedom is also strong, but Friedman, while brilliant, was somewhat more ideological, simplistic and one-sided.
Fortunes Formula: The Untold Story of the Scientific Betting System That Beat the Casinos and Wall Street, by William Poundstone. Ties together the common quests of pioneers of information theory, investors, and degenerate gamblers and mobsters.
Fiction
The Razor's Edge, Somerset Maugham. A case study of ways in which different types of motivation can dominate different people in constructive and destructive ways.
The Jewel in the Crown, by Paul Scott. In each chapter, a different character relates a Rashomon-like point of view of a shocking crime, and through them a mystery is explained, and a complex portrait of India's many faces is revealed.
I have a soft spot for satirical Brit-novels, like Vanity Fair, The Way We Live Now, and Wodehouse.
Biography
Buffett: The Making of an American Capitalist, by Roger Lowenstein. An insightful biography of the greatest living investor, or maybe of all time.
Gates, by Manes and Andrews, is also very good.
Science
Dancing Wu Li Masters: An Overview of the New Physics, by Gary Zukav, and The Tao of Physics, by Fritjof Capra, on the unbearable elusiveness of reality. If a particle is just a probability density function of potential interactions with other particles, does the substance of reality reside in the particle, or in the interaction?
Grammatical Man : Information, Entropy, Language and Life, by Jeremy Campbell. Is the substance of reality matter or information, or are they the same? If biology is applied chemistry, which is applied physics, which is applied mathematics, maybe mathematics is the study of formal systems, which is a subset of things that can be computed and decided, ie computer science, which is a subset of things can be expressed and communicated, ie information theory.
The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics by Roger Penrose, a massive grand tour of scientific knowledge and big questions of the late 20th century. When you get done with that, go for the sequel, The Road to Reality: A Complete Guide to the Laws of the Universe... I haven't yet, but good luck!