hmm..I'm sure I wrote something earlier, but all I remember was my Creative Writing class in college; I was the only prose writer, everyone else wrote poetry..bad poetry..;) I remember finally cluing in that my teacher was in a particular studies department, so I wrote a sci-fi (don't I always..;) story that catered to her specialty, and changed my grade from a 'c' to an 'a'..folks, know your audience, and write to that..;)
First adult book? gad..I still have some of my first adult..oh..you mean mature..;) hmm..Dickens, probably, I think I read some 'adult' sci-fi the World Inside was one of the earliest..though I admit to reading a book called Huckelberry Hashimoto which had 'adult humor'..made a lot more sense once I was old enough to figure out the dirty words..;)

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)