This is wonderfully written book of apologetics. The best writing grows not necessarily out of right thinking, but out of clear thinking. You may not agree with its central thesis or the arguments expertly outlined and colourfully illustrated in the first half of the book, but if you, like me, get an almost sensual pleasure from good writing and clear thinking then you will certainly get your fix from this book. That is the first thing to say.
If you are reading this then you have directed your eyes to this page to read these lines. Whether you will find yourself conducive to Lewis' reasoning depends on whether you think my observation above is possibly a miracle. That is, whether you believe in free will. The very thought process and resulting choice that led you to read these lines is a product of your mind. If you think your mind is equivalent to your brain then you are a machine and I would ask you not to read on: you cannot understand what I have to say: please desist. But if you have been following my argument so far then I think you will have to admit that your mind is something quite special; it possesses reason. Reason is the divine spark in us according to Lewis, because it is what makes the difference between man and brute a difference of kind and not degree. Nature does not explain itself, it just is. Through physical science man has discovered some of the laws of nature, some of 'how's'. But physical science will never give you answers to the 'why' question, the question of meaning. And yet this question is implicit in the human mind, in reason- finding reasons, not just explanations, but justifications as well. It is this basic fact of human spirituality that corresponds directly to a reality according to Lewis: the supernatural ground of the natural, the first cause, the unmoved mover and so on.
If you accept all this as reasonable, if you find it meaningful, then you will enjoy the second half of the book, which discusses some of the Christian miracles. This work is not an attempt to verify miracles. It is simply a groundwork intended to clear the mind of pre-reflective prejudices, to allow room for the possibility of miracles. It serves its purpose admirably in this respect. Even if you are not convinced that miracles have happened you will be convinced at least that they are just as rational as not. This work broadens the mind.