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.
You know, you guys really make me want to read some Hardy to see what all the bloody fuss is about.
Okay, it's time to play Guess the Book! I read a book in ninth grade for English class. All I remember is that it was about a man who fell in love with a woman in a painting, and I think he ended up going inside the painting, maybe? And there was water involved, like an ocean. And I seem to recall the word "Alhambra." It may have been the name of the bar where he talked about this woman to the bartender.
I randomly recalled it the other day, and now it's bugging me that I don't remember what book it is.
Love Roddy Doyle.
Me too! I must've read Paddy Clark, ha ha ha a hundred times when I was a teenager. I havent looked at it in years though. Maybe it's time to root it out again now I've made it through my backlog of books.
P-C, could it be Portrait of Jennie?
YES THAT IS TOTALLY WHAT IT IS OMG.
And I knew the main character's name started with an E: Eben!