I can't believe no one mentioned Russell Hoban, author of both Bread and Jam for Francis, and Riddley Walker. (Among other things for various ages of reader.)
Literary Buffistas 3: Don't Parse the Blurb, Dear.
There's more to life than watching Buffy the Vampire Slayer! No. Really, there is! Honestly! Here's a place for Buffistas to come and discuss what it is they're reading, their favorite authors and poets. "Geez. Crack a book sometime."
Wow, Russell Hoban is a great example. Again, I had no idea he wrote anything but children's books (my favorite was always Harvey's hideout), but I don't read a lot of fantasy or sci-fi. Thanks, Jesse!
Riddley Walker is a fairly amazing book, but even more so when you remember that it's the same Russell Hoban.
I don't think of Oscar Wilde as a children's author, but he had a story in Olive Beaupre Miller's "My Book House: Through the Gate" (aka Volume 4). He contributed "The Selfish Giant," about a mean giant who wouldn't let the children play in his beautiful yard.
Donald Westlake has written mysteries, caper novels, satire, science fiction, and apocalyptic fantasy under his own name. And then there's even more stuff he writes using a pseudonym.
I was gonna mention Westlake!
Uh, Ted Hughes wrote poetry and children's books (very successfully).
I don't think of Oscar Wilde as a children's author, but he had a story in Olive Beaupre Miller's "My Book House: Through the Gate" (aka Volume 4).
He wrote quite a few fairy tales for children (mostly for his own sons, but he didn't hesitate to get them published either).
Also, George Macdonald wrote adult fantasy novels (Phantastes, Lilith, and one I can never quite remember the title of), straight-up non-genre novels, religious essays, and fairy tales.
I had a book of Oscar Wilde's fairytales when I was a kid. And Roddy Doyle is another author who does grown-up books and kids' books, the only other one I can think of that hasn't been mentioned, anyway.
In other literary news, I finally finished The Algebraist today. It only took a year. The only other book that's ever taken that long is Tess of the d'Urbevilles. Which made me want to die, incidentally.
Love Roddy Doyle. And not just because we both like soul, although that was the beginning.
I don't know if this counts for what you're after, but Iris Murdoch wrote both novels and books on philosophy.