I first discovered Roald Dahl's adult books when I happened to see a TV show called "Tales of the Unexpected" based on Dahl's short stories for adults. In fact, Dahl himself introduced some of the stories on the TV show. It was so good, so spine tingling, and I just had to go out and read the books.
A couple of my favorite short stories are "Royal Jelly" and "Skin." On the TV show, Royal Jelly was SO well done. I have to try and find that TV series on netflix or something.
I personally find many of Dahl's children's book a little bizarre, but looking at the adult short stories, I can see where his writing interests certainly are, and "bizarre" is definitely on his radar.