A lot of good stuff mentioned. My recommendations:
Anything by John Varley. Why he isn't far more well known and popular with dozens of film adaptations of his novels is beyond me. Good, "hard" sci-fi. Not necessarily kids stuff - lots of frank discussion of some uncomfortable topics. The series of Red Thunder, Red Lightning, and Rolling Thunder are his homage to Robert Heinlein, and probably a good place to start.
To Kill a Mockingbird - it's one of those books that always gets assigned in school - each of the 3 years that I spent teaching English I assigned it to at least some of my classes. And I always loved reading it every time.
Mark Twain is always good. I don't think a lot of people realize just how subversive he is. I can't even imagine how disappointed he would be with the US today.
Clive Barker is someone that more people should pay attention to. I think a lot of people dismiss him as just a horror writer, but he actually is more a fantasy writer than a horror writer (though there is some terrifying scenes in the fantasy novels). I recently read Imajica (published in 2 volumes). It is an amazing feat of world building and storytelling. Again, not kid friendly. However, the book The Thief of Always is an equally wonderful story that is totally appropriate for the 9 year-old and up crowd. I read it a few years ago and regretted that I hadn't read it sooner because I would have loved it so much when I was young.