The Screwtape Letters is probably the lightest and the easiest to get into. It is a sort of satire, comprising a collection of letters from a mid-level manager in hell to his nephew Screwtape, a rookie tempter assigned to a human circa WWII London. It is celebrated for its insight into the roots of sinful behavior - within the context of Lewis' experience of (probably Anglican of some sort) protestant Christianity, and its readability. He is a very clear and entertaining writer.
A Grief Observed is a very personal and insightful account of Lewis' attempts to come to grips with the death of his wife, again within the protestant lay-christian perspective. Not sure that it provides great guidance on approaching / dealing with grief for the non (or divergently understood) Christian, or for the Christian (Lewis seems to have been quite serious and cerebral about his faith... after he came to it - converted as an adult).
The Problem of Pain is pretty much just what it says: a study of pain in Lewis' Christian context and in consideration of the omnipotent/benevolent deity.
Another book, I don't have but would recommend it, is The Four Loves, which examines the implications of different Greek words used (in Xtian gospel/epistle?) to express some aspect of love - filial, erotic, unconditional, and so on.
The one I'm currently reading also appears to be good: it's a collection of essays, lectures, letters and op-eds on a variety of topics. The reason I would recommend it is that the entries are short and sweet. So one (you!) could get a taste for his writing without too big a commitment.
Reading this one (and others), I get a little annoyed at some of his conceit with respect to points of faith or scripture that he uses as major premises in logical operations and exposition, but usually cannot fault his writing or (the remainder of) his reasoning.