I read this many years ago as a young teenager, and I'm sure I still have my paperback copy of this book boxed up somewhere.
Go Ask Alice is about a teenage girl who gets addicted to drugs. It is written in the form of her diary, and as a teenager myself at the time, it was impossible not to put myself in her shoes, experiencing everything she was going through, as her life fell apart around her.
It's not an easy read, and has haunted me and stayed with me ever since. Probably a good book for parents to give to their teenage children, in the hope of them not taking things too far with drugs.