George Bernard Shaw by G. K. Chesterton (1909)
gutenberg.org
gutenberg.org
I am not sure Tolkien's books are accurately described as children's books. The Hobbit perhaps, but not the Lord of The Rings IMO.
But Chesterton's non-fiction books, The Everlasting Man, Orthodoxy, What's Wrong With the World and Heretics are some of the best and most insightful books I have ever read, and they really took me by surprise because I did not expect them to be what they are. They are full of speculation, and misunderstandings, but they are also full of very deep insights into humanity.
I do, however, think the fiction of Tolkien is historic and unsurpassed, nothing written since really comes close IMO.