Up, Down, All-Around Stitch Dictionary by Wendy Bernard of Knit and Tonic is a fantastic and amazingly helpful book; every knitter should own it. It clearly and beautifully explains how to knit over 150 stitches and how to knit them top down, bottom up, back and forth, and in the round.
The book is bound on the outside, so it sits perfectly on the shelf. However, it's spiral bound on the inside, so the pages keep easily open while using it as a stitch guide when you are knitting. Brilliant!
What I enjoyed most about this book is it gave me the clarity and more importantly the confidence to write my own patterns. I have never written anything complicated, granted, but this book has helped to understand the architecture of knitting which I think is a good foundation for pattern writing.
Bernard has also released a new book, The Knitting All Around Stitch Dictionary. I don't have this one yet, but it's definitely on my list.