I was also surprised...Not by the book, or the content--this is fact was great reading. My surprise was that Lewis enjoyed my great-great-uncle's writing! He mentioned Rider Haggard part way through, which actually made me jump!
Lewis can write, there's no doubt about that. I have read many different genres of his. Narnia was one of the series of books that got me hooked on reading in the first place. Even at 50+, I will happily sit down and read the whole set.