Well I don't know how much philosophy you've read so I'm not really sure if I should recommend anything difficult or just stick to easier stuff. I guess I'll just try to mix it up.
The Philosopher's Toolkit by Julian Baggini and Peter S. Fosl: This is a very readable and easy introduction to many of the abstract concepts used by philosophers in argumentation and critique.
The Blackwell Guide to Epistemology by John Greco & Ernest Sosa: This is a good introductory text to the various arguments and ideas in epistemology (theory of knowledge). If you're interested in that subject check it out. The only bad thing is that the book gets a bit dense and complicated as it goes on because it includes articles written by contemporary philosophers who probably assume that the reader already has some background in the subject.
An Enquiry Concerning Human Understanding by David Hume: This is pretty much a seminal text that I think anyone genuinely interested in philosophy should read. The influence of the ideas found in this book have been pretty widespread. Not only philosophy but the scientific, empiricist understanding of the world owe a lot to Hume's ideas. And it's pretty readable for an 18th century text.
Rationality in Action by John Searle: Searle is probably one of the most clear and accessible writers in contemporary philosophy. The book deals with practical rationality and free will. He makes some pretty good arguments against traditional ideas in this one.
Walking the Tightrope of Reason by Robert Fogelin: This one deals with issues of skepticism, knowledge, reason, etc. It's a pretty interesting read but I think some of the guy's conclusions are kind of weak. Anyway, I'm recommending this because it's a decent introduction to the issues I mentioned and it's understandable.
Principia Ethica by G.E. Moore: Pretty much a classic. The book deals with metaethical and normative ethics issues. This is where Moore made a lot of strong arguments against prevailing ethical theories.
Empiricism and the Philosophy of Mind by Wilfrid Sellars: Another classic. This is basically Sellars arguing against sense-datum theories. Don't read this unless you've familiarized yourself with the whole sense-datum concept and writers like Hume and the logical positivists.
The Conscious Mind by David Chalmers: Apparently this has been all the rage in the philosophical community for the past few years. The book deals with consciousness and argues against materialist views of consciousness. I'm a little skeptical about the whole thing but whatever... Anyway, it's very interesting if you're fascinated by consciousness and if you find yourself utterly baffled by the fact that lumps of grey matter can produce experience. The book gets quite technical in a lot of parts so be warned.
