Finished Don't Tell the Grown-ups: Subversive Children's Literature by Alison Lurie.
It didn't really end up being what I thought it would. There are mainly essays about children's writers & illustrators (Kate Greenaway, Mrs. Clifford, Ford Madox Ford, Beatrix Potter, E. Nesbit, James Barrie, Frances Hodgson Burnett, A. A. Milne, J. R. R. Tolkien, T. H. White, Richard Adams & William Mayne). These were quite good; I didn't know much about most of the authors' personal lives.
Then there were a few essays on fairy tales, adults reading children's literature, and "the folklore of childhood." That was what I was really expecting the whole time. Upon looking at Amazon, I see that the author had another book called Don't Tell the Grown-ups: Why Kids Love the Books They Do. Since I originally decided to get this book from the library after finding it in a used book store, perhaps I conflated the two. Although it seems just as likely that the one I read is a newer edition with a different subtitle.
It's a bit dated now; it would be interesting to see what the author has to say about the resurgence of children's literature (if that is really what Harry Potter et al. is). I've never given up reading books aimed towards young adults -- some of the best stuff is there.