Quoted for emphasis on all points. Mere Christianity is a very UNphilosophical philosophical book. By that I mean that it is both accessible and well-reasoned, but without a lot of the superfluous BS or overly technical writing that many philosophical books contain. I would put his writing style close to that of a Christian version of Bertrand Russell in the readability regard. It reasons with conviction, and I love that in a writer.
I will also say that Screwtape Letters is a great book for both Christians and non-Christians. Great message, great writing. And I believe Mere Christianity to be a very good book on what true Christianity is supposed to look like, in addition to being an intro to apologetics. In my opinion that is probably its greatest strength, however I encourage everybody with an interest in the philosophy of religion to read it on a variety of grounds.