Typically, i'd not post a review of a book anywhere but on the page i purchased the book from (or not at all if purchased in-store), but this one demands an exception. Mere Christianity is quite surely the most approachable book i've read on Christian living (outside the Bible itself, of course), and not only that, on Christianity in general.
The first portion of the book is devoted to making sense of a Spiritual Being from the physical standpoint. For the unbeliever, it brings logic into the idea of believing in a creator; to the Christian mind a fresh look at who God is, which, in turn, makes God much easier to understand and draw closer to.
He lays no scientific claim on Christianity (as he says, the existence of God can't be proven any more than it can be disproven by science). His perception, instead, of morality, the driving force to look past self in order to care for others, is how he proves God--and he does so impressively well. At times you would even feel God is the truly tangible and man the intangible (which is closer to reality, and he discloses such exquisitely).
After establishing why faith in a creator is not only plausible, but also logical, he then sets out to point towards the Judeo-Christian God. His goal, as he points out, is to make Christianity approachable without introducing any of the denominational differences (thus the title, Mere Christianity). Again, he does this rather well, though there are times that it comes off as a particularly Anglican text (after all, he was a member of the Church of England), sometimes even Lutheran, though the differences are trivial. All-in-all, it is a very non-denominational book.
When he gets into what a Christian life looks like, he has a way of skewing things just enough to make it new and applicable, though he doesn't lose the original intent. His metaphors are spot-on every time, even when you don't want them to be. When speaking on "good behavior," he answers why some Christians can be less "good" than some non-Christians, and that was something that had been a bothersome question for me.
His views on faith will challenge your own, but in such a way that it will bring it to a greater sense of resolve.
Through all of this, he keeps God on a very intimate level, which is a rare feat. When most try to make sense of believing in not only a god, but in the God, they tend to leave out that He's a very personal Being. This book is a great exception in such that it makes the case for God, it makes a case for Christ and why Christ died for us, what grace means, and reinforces the notion that God loves us, sinner, saint, and everyone in between.
Easily one of the most eye-opening books i've read on Christianity, though it presents few things that are not (or at least should not be) commonplace to a Christian. It does, however, bring these things up poignantly and in a way we can relate to.