The History of YA Literature
I have actually read the latest issue of The Horn Book. It includes an interesting article entitled The Outsiders, Fat Freddy, and Me by Patty Campbell, a former YA librarian who speaks on YA literature. Her article describes how the YA field took off in the 1970s and the sense people working in YA lit at that time felt of being on the cutting edge, of doing something subversive and dangerous. At that time YA lit was not kids' stuff. They were definitely talking about readers in their later teens.
I found this fascinating because so many adults--at least the ones I know--feel just the opposite about YA fiction. They view it as a lame genre, written by second-rate people who aren't capable of writing for adults. In my experience the people who feel this way haven't actually read much YA lit, if they've read any at all. In fact, I feel pretty safe saying that most adults don't have a clue about YA maturity of content or sophistication of writing styles.
Campbell's article is worth looking up if you want to know more about the YA field.