This was Leanne Payne's very first book, and she presents Lewis's writings as if they represent the epitome of orthodox Christianity. In fact, she often does the same in her later books as well.
Unfortunately, Lewis held some seriously heretical doctrines. For instance, he believed that macroevolution was a perfectly valid explanation for the origin of life. He also didn't believe in the inerrancy of Scripture. And he thought that the story of Adam and Eve might well just be a myth, not something to be taken literally. Some Christians find the themes of "white magic" that run through many of his books to be problematic as well, believing that they have occult undertones. In addition, apparently as a university professor, he was often known to go out with his students and get drunk. To learn more about the many heresies that Lewis believed, you can do a Web search for this article: "C. S. Lewis — the most beloved heretic."
And to make matters worse, Lewis married a divorced woman, Joy Davidman, in 1956. He had sought the blessing of the Anglican Church, but the Bishop of Oxford refused to marry them because Joy had been divorced. Since the Anglican Church did not condone divorce, the bishop would not give them his blessing. To be fair, Davidman's first husband William Gresham had been married before, so one could make a case that his marriage to Davidman was not a real marriage. Still, Lewis flat-out ignored the advice from the Bishop of Oxford and apparently found another bishop who would give him and Joy his blessing. At the very least, you could say that Lewis married a woman who had been living in adultery with another man for over a decade. Does that really sound like God's will to you?
Now, for the book itself. Much that Lewis wrote was insightful, and Payne evaluates various themes that run through his works. This is a short book, and not a particularly difficult read. It's often very enjoyable, helping the reader understand various key truths about the Christian walk.
What the book does, it does well. However, it's very odd that Payne offers a sharply critical critique (which is well-founded) of some of Charles Williams's views in the appendix, but shows nothing but uncritical adoration toward the ideas of Lewis. The fact that she fails to warn readers about the many serious problems with Lewis's views is a major blind spot of this book. I don't know whether she just failed to do her homework, or actually knew about all the problems with Lewis but simply chose to whitewash them (which would be engaging in, as Payne would say, the vice of sentimentality). I suspect the former.
Anyway, if you're a reasonably mature Christian and already understand the major flaws in Lewis's thinking, you might want to check out this book. But if you're a baby Christian, it's probably best to just stay away.
Overall, Payne's half-baked scholarship in regard to Lewis makes it kind of hard to take this book seriously. I mean, how on earth can you expect anyone to read a book that you've written when you haven't even bothered to do thorough research?