I have been knitting for a little over two years and I have collected a lot of knitting books. The one I use the most is Knit Fix by Lisa Kartus. I always keep it close by while I'm knitting. So far I haven't had a problem with my knitting that isn't covered in this book. I love it!
I have all of Sally Melville's books and recommend her because she is such a great instructor. However, I will say that I don't like every single pattern, but there are enough patterns that I do like to feel justified spending the money for the books.
I recently got Fitted Knits by Stefanie Japel. I haven't actually started anything from this book yet, but the projects are beautiful and designed for minimum finishing (my favorite kind of knitting).
No Sheep for You is next on my list and I really want to get The Pleasures of Knitting by Ann McCauley because I absolutely LOVE the Peri's Parasol Pullover (check it out on KnitPicks under patterns/sweaters). I MUST have that sweater -- someday!