It's been 20 years since i seriously read philosophy/lit crit, but if you want a gateway book that will show you the power of stuff like that, I'd recommend "Practical Ethics" by Peter Singer. It set off a firestorm of criticism from advocates for the disabled, you might recall. It's a great read that can stand on its own without a grounding in classical philosophy.
Everyone in this thread has been recommending very specific fields of philosophy for people who want an intro to read. Like econ, or ethics. The others have gone off talking about continental theory.
If you want an introduction, read an introductory book, "Modern Philosophy" by Roger Scruton is a good beginning, he covers the groundwork of most modern philosophy (continental philosophy and analytic philosophy). Once you get more into philosophy, you'll find that during the 1920's to about 1930's, a large gap between so called "analytics" and "continentals" began to form. Analytic philosophy essentially is about things that can be proven a priori, or through pure reason, without any sort of empirical data, or empirical anything. They talk about philosophy of mathematics, logic, linguistics etc. Russell, Wittgenstein, Frege, Quine, are analytics. The continentals are people who began to be increasingly interested in social phenomena, these people are like Lacan, Zizek, Derrida, Hegel, etc. and the "continental tradition" encompasses existentialism (which I feel doesn't actually define the so-called "existentialists", but simply denotes a common train of thought/interest in the self that was common for a certain time period). Continentals usually are pretty interesting but they usually use excessively complex sentence structures and generally introduce their own lexicon to describe phenomena that they observed, which leads to lots of debates about what they actually mean.