I have to vote for (a) Vogue Knitting and (b) Knitter’s Handbook by Montse Stanley. The are both technique and reference books (though Vogue has a few projects at the end – sort of an afterthought). And if you can only have one book, Vogue Knitting is it because the organization is so clear. I love Stanley’s style, but specific answers are sometimes a bit tricky to find. They’re all there, though.
The other books mentioned aren’t bad, and will take you through beginnerhood, but Vogue and Knitter’s Handbook will be your trusted friends for your entire knitting life. If you want to move beyond scarves and simple sweaters, go for the good stuff.