I can't believe no one's mentioned Lurlene McDaniel as part of the Dead Teenager category! When I was 11 I read every book of hers in the library: Six Months to Live, and it had sequels! Was one of them Six More Months to Live? That's can't possibly be the actual title, right? There was one called I Want to Live, though. And either she or someone very similar also wrote books about kids with runaway siblings who went off to The City and Lived On The Streets and also Got Hooked On The Drugs and even Stole Things Sometimes.

Oh, Lurlene McDaniel. So much melodrama in such short books!

I also read pretty much all of Agatha Christie's oeuvre when I was in junior high. Then I discovered Dorothy Sayers, and the world got immeasurably better.