Rather than commenting on the book any further, I'll let Lewis speak for himself. The passages I've chosen illustrate his anger, sadness, doubt, and finally his peace.
"Meanwhile, where is God? This is one of the most disquieting symptoms. When you are happy, so happy that you have no sense of needing Him, so happy that you are tempted to feel His claims upon you as an interruption, if you remember yourself and turn to Him with gratitude and praise, you will be--or so it feels--welcomed with open arms. But go to Him when your need is desperate, when all other help is vain, and what do you find? A door slammed in your face . . ."
"Not that I am (I think) in much danger of ceasing to believe in God. The real danger is of coming to believe such dreadful things about Him."
"Her absence is like the sky, spread over everything."
"It is hard to have patience with people who say, 'There is no death' or 'Death doesn't matter.' There is death. And whatever is matters."
"Kind people have said to me, 'She is with God.'" -- "Talk to me about the truth of religion and I'll listen gladly. Talk to me about the duty of religion and I'll listen submissively. But don't come talking to me about the consolations of religion or I shall suspect that you don't understand."
"Your bid--for God or no God, for a good God or the Cosmic Sadist, for eternal life or nonentity--will not be serious if nothing much is staked on it. And you will never discover how serious it was until the stakes are raised horribly high, until you find that you are playing not for counters or for sixpences but every penny you have in the world."
"What do people mean when they say, 'I am not afraid of God because I know He is good'? Have they never been to a dentist?"
"I have gradually been coming to feel that the door is no longer shut and bolted . . . Perhaps your own reiterated cries deafen you to the voice you hoped to hear."
"Two widely different convictions press more and more on my mind. One is that the Eternal Vet is even more inexorable and the possible operations even more painful than our severest imaginings can forbode. But the other, that 'all shall be well, and all shall be well, and all manner of thing shall be well."
"Lord, are these your real terms? Can I meet H. again only if I learn to love you so much that I don't care whether I meet her or not? Consider, Lord, how it looks to us. What would anyone think of me if I said to the boys, 'No toffee now. But when you've grown up and don't really want toffee, you shall have as much of it as you choose'?"
"Heaven will solve our problems, but not, I think, by showing us subtle reconciliations between all our apparently contradictory notions. The notions will all be knocked from under our feet. We shall see that there never was any problem."
"We cannot understand. The best is perhaps what we understand least."