I taught myself the basics from a Stitch 'N Bitch book, but I don’t remember how much detail she got into about things like when to use what kind of decrease. But it was good for the basics, especially her drawings explaining how stitches are supposed to sit on the needle.
Principles of Knitting by Hiatt is a nice meaty reference book that covers almost everything you could ever need to know. That’s the only general reference book I have, but I also have access to a library where I can get any others I might want. The others I have are design-oriented, like Knitwear Design Workshop, which could be helpful for helping you understand why to do some things, but aren’t really meant for general reference.
Knitting in Plain English by Righetti is another good option. A lot of people like Vogue Knitting’s Ultimate Knitting Book. There is a book out there called Increase, Decrease, which is pretty cool but probably way more than you really need unless you’re going to start designing patterns.
Techknitter’s blog is a great online reference, although I know she does get into way more detail than some people want. She just has a very nice index to make it easy to find what you’re needing.