I highly recommend "Two Dogmas of Empiricism" by Quine. It's just a short little essay, but it's quite accessible (as far as these things go) and was highly influential (and is really fun to read too).
+1 Reasons and Persons. The Personal Identity section in that in particular is wonderful! It's a really good book because it kind of stands alone -- you don't need to be very well-versed in philosophy to get the arguments if you read carefully.
