http://www.philosophybites.com/
Check out Stanford Encyclopedia of Philosophy, which is free and edited by active research philosophers on pretty much every major topic and figure.
Naming and Necessity by Kripke is about reference and the causal theory of reference, but it assume a decent amount (in case that is what you were asking for specifically).
The Very Short Introductions series have some really good booklets if you want quick ways to jump into a topic.
A few other books:
B Russell The Problems of Philosophy. OUP (dated, but a classic general introduction)
Simon Blackburn Think. Oxford (nice general introduction)
Tim Crane The Mechanical Mind. Penguin (Nice introduction to phil of mind)
A J Ayer Language, Truth, and Logic. Penguin (dated, but a classic)
D Hume Enquiries
T Kuhn The Structure of Scientific Revolutions. Univ of Chicago Press