I'll second
Knitting Without Tears by Zimmerman, if you are looking for a helpful reference. I also like
Knit Fix
as another resource book.
I have lots of pattern books, more for inspiration than anything else. It's tough to recommend books, because it depends on your style and aesthetic preferences, as well as the things you like to knit.
One helpful thing I discovered is to check on the Knitty or Ravelry discussion boards or Amazon for reviews of books, as well as on the author's or publisher's sites for specific books BEFORE I buy. I'm looking for the corrections for patterns. Much easier to be warned about a problem before I start, rather than knitting away happily for a week or 2, only to find an error in the pattern.