I have "Vogue Knitting, the ultimate guide", "Knitting in Plain English", "The Big Book of Knitting", and a few others.
I use the Vogue book the most for basic how to information from knitting stitches, seaming, and more. Knitting in Plain English is the only one I have read cover to cover. It's full of ideas on how to do things... more like talking to a long time knitter. It's full of helpful hints and ideas, and at least when she has a strong opinion (and she has several) she admits it. I don't find myself using the Big book very often.
I love the Barbara Walker stitch pattern books. Has anyone compared these to the new Vogue Stichtionary books? I haven't seen those yet... might want to compare first.
I would like the Kenzel Modern book of Lace knitting volumes 1 & 2, but that has to wait a bit. The best price I have found so far on these is at elann.com (better than Amazon even before the bonus you get when buying yarn).
There are also a variety of books on finishing techniques, resizing or making your own patterns, any of the Elizabeth Zimmerman books that encourage you to enjoy a bit of freedom with your knitting... Maybe someone else can tell more about those or other books I'm forgetting, or give you alternate opinions on the books I have listed.
Edited to add: This is a dangerous thread. We need a warning sign! By time I wrote my post there were two others above me that include a few books I don't have... and of course now I want to check them out!