The most accessible guide to Stoicism that I've ever found was recommended by Derek Sivers, and written by William Irvine "A Guide to the Good Life".
http://www.amazon.com/Guide-Good-Life-Ancient-Stoic/dp/01953...
I really highly recommend it myself. A bit of history of Stoicism, but also a lot of practical advice about how to put it into practice in modern times. (Always going back to the key Stoic thinkers.)