If you could insert a chapter about your own ghost, how would it read? What thing is the most important thing in your life? What idol are you prepared to protect and pursue at all costs?
Nothing in and of itself is good or bad (how can the love of a child be wrong) but when that thing takes the place of God, it causes us to miss the mark (i.e. sin). Therefore, the challenge that Lewis gives us is to see those idols in our life that draw us away from God so that they can be put in their proper place.