'It' by Stephen King
It was the first Stephen King book I read, not long after my 16th birthday. From that moment on, I was hooked. I've read almost everything he has written, and own the entire collection.
Such a shame he hasn't been writing real horror these last couple of years... I'm still looking for a worthy substitute... :0)
He's a great writer! I enjoy his more recent books (not so much horror!) but he does horror so well, too.