One of the things that my husband gave me for our anniversary is a marvelous knitting book, it's very new, just off of the presses, so to speak, The Best of Vogue Knitting 25 years of articles, techniques and advice. There are 25 articles out of Vogue Knitting over the past 25 years and it's fantastic, I really love it!
One of my all time favorite books is one I've had since right after I began knitting, Scarf Style, I also adore the other 2 "style" books (Lonnie surprised me with these, too!), Wrap Style and Lace Style. My goodness, I could go on and on!
Oh, and there is Best of Interweave Knits that is to be released in the fall.
Everyone has such eclectic tastes, I'm sure you will get many answers, I have learned a lot from the Vogue Knitting book that Lonnie gave me last month already. You might want to look at the books at KnitPicks, they have a great selection and you can usually glance few some pages of each book.