This is a great book.  Lewis tackles the issue of pain from a very modernist perspective.  He doesn't bother spending a lot of time on how we feel about pain or whether it's nice or not.  He assumes that the world is a rational and objective place, that there are actual objective truths that aren't at the whim of our personal desires or a multiplicity of personal preferences.  He starts with the way that God appears to have designed the world and logically deals with the reality of pain  in a real world that is seriously broken.

While he goes off on some tangents that I don't personally find very compelling or even necessary (the near-last chapter on animal pain, to be specific), overall the book is right on target.  Much of the theology here is hinted at or treated in passing in his other works, so you may find some of this to be pleasantly familiar.  

If you wish to have some intelligent answers when people ask you how you could love and trust in a God that allows suffering and pain in our world, this is a book worth rereadingr again from time to time.