On C S Lewis, by Owen Barfield.
Barfield was one of Lewis's longest and closest friends, an Inkling and one of the companions on the walking tours of the 20's & 30's. Lewis praised his intellect and declared he was much indebted to him for opening out ideas of imagination to him early in his career. I have much wanted to like Barfield, and he seems a decent individual, but I have never much warmed to him. I have never found anthroposophy in the least persuasive, too reliant on mysticism and even a bit occultic. He seemed to my naive mind a bad influence on Lewis, though I suspect Jack was up to the task of resisting anything heterodox.
I did learn some things from this collection of nine essays. I was surprised to learn that Lewis's expression and tone of voice changed little while conversing. He was not dramatic. Barfield denies he was much influence on Lewis after the initial years, and despite Jack's characterisation in Surprised By Joy of "The Great War" as an ongoing intellectual battle, Barfield claims they never discussed the items of contention after the 1930's at all. He concluded that Lewis kept up the discussion himself, in his own thought, not needing much assistance. This would be consistent with the view of the other Inklings, especially Tolkien, that Lewis liked many books which were not all that good, because he supplied a good deal of the imagination and logical argument himself.
That's about it. I gained something from the 1st and 2nd essays ("C. S. Lewis" 1964; "C. S. Lewis in Conversation" 1971) and the 8th ("The Five C. S. Lewises."), but the rest were of little interest. Unless you are already much taken by philosophy, and its discussions of precise definitions of unusual terms, or the influence of Coleridge's thought - or if you are well-enough versed in anthroposophy that its topics come naturally to you, I don't recommend the book.