Stochastic processes

The math which goes into stochastic theory is key and fundamental; this book is a very readable introduction to the material. Where most books just have the equations (and typically in the tersest form possible), this book makes an effort to explain what is going on "in words" and through some examples. If you are in a class or working with material where a firm understanding of stochastic theory is necessary, this is a good book to help you get up to speed. This is definitely not an advanced-level book or completely up-to-date, but the basics haven't changed. If you can find it used, it is cheap, too.