Interweave's Knitter's Companion
The Knitter's Handbook
I check these out of the library every now and then:
The Kniting Answer Book
Harmony Guide Books
I'd definitely check your local library to see what you can borrow first. I've borrowed books I thought I had to have only to discover I didn't feel like they were worth buying. Some I liked so much I did buy my own copy. Some, while I liked the book, I didn't buy since I can check them out as often as I need.