Recently i ran across a small paperback by Elisabeth Elliot called The Liberty of Obedience. It's a short collection of a few articles written while she was living with the Aucas/Waodani in Ecuador, observing the way they lived and contemplating the nature of the gospel. (If you haven't heard the story of the Saints and the Elliots and the Aucas/Waodani - you definitely need to do a web search on this and find out - it's an incredible story.) It's a short read but really good. She talks about what it really means to "avoid the appearance of evil," what true worldliness is, and how Christian maturity involves moving past a simple (but often long) list of rules to a deeper ability for discernment. If you can get a hold of it i highly reccomend it.