What were the first adult books you read? I realised in a recent conversation that I can still remember quite a bit about my parents' books, in many cases down to which shelf they were on, and that I still recall some of the earliest adult books I read.
In early primary school I just ploughed through everything in the school library, but some time around ten or eleven I started migrating to adult novels, first from my parents' books and then from the local library (which fortunately had an excellent science fiction and fantasy holding).
I'm not sure which books I read first, but the following are among the ones that I remember as being confronting or influential:
* I read Lord of the Rings when I was about ten, and Tolkien was a huge influence on my sister and me. Lord of the Rings is not "adult" in the way something like A Song of Ice and Fire is, but it's still a huge step up from Narnia, which it pretty much purged from our affection.
* Despite being about rabbits, Richard Adams' Watership Down is not a children's book at all and I can remember finding it quite confronting. And the opening chapter of Shardik, with a bear being driven by a forest fire, left a huge impression on me. I think this was the first book I read from my parents' "general fiction" shelf behind the television. (I think I had some idea of proceeding alphabetically!)
* I read through my father's shelf of classical literature (Penguin Classics), which included Plutarch and Herodotus and Livy and Tacitus, but the writer who made the greatest impression on me was Thucydides. The tragedy of the disaster in Syracuse was somewhat overwhelming for an (11?) year old.
* Henry Treece and Rosemary Sutcliff wrote historical fiction for children but also for adults, so that was an almost accidental progression. And I must have started on Mary Renault early, as I remember the opening castration in The Persian Boy being a bit of a shock.
* My father's science fiction collection included The Fifth Head of Cerberus, Gold the Man, the Dorsai trilogy, and so forth.
I never really read "young adult" books. The library did have a section of those, but the sf and fantasy was all in the adult fiction.
When I got to high school, I worked out that my $10/week allowance would stretch to a book a day, buying $2 paperbacks from Tyrell's secondhand bookshop in Crows Nest on my way home. And since I was exempted from having to do any work in maths classes I had a huge amount of time to read, and ploughed through a couple of dozen books a week in junior high school.
My parents never put any controls on my reading, at least not that I noticed, and it would probably have been impossible anyway.