C.S. Lewis's autobiography/spiritual reflection Surprised By Joy is, of course (it being Lewis), a good read with myriad insights into his thought process of becoming a Christian. This is really quite a good autobiography, albeit incomplete from a truly autobiographical point of view--Lewis only puts in things that are related to his spiritual growth, but that does include sections on his family that I found very insightful and occasssionally entertaining. I didn't realize he didn't have a good relationship with his father, for example, until I read this work. He chronicles his journey from leukwarm faith, to atheism, back to belief, and then, finally, to Christianity, for his conversion (Or "reversion") to belief was just to that--to belief, not to Christianity specifically, although he has reached that point by the end of the work.
If you're a fan of Lewis, this is required reading, and if you're just interested about learning more about the man who gave us The Chronicles of Narnia and Mere Christianity then this is also good for you. It's written in the same accessible style as Lewis's other works and can be found in several editions, although for awhile it was harder to find the newest Harry Potter . Would make an excellent Christmas gift for the reader on your list, especially the Lewis fan, since most people tend to overlook this book in favor of his other apologetics works.