Respect the Spindle by Abby Franquemont: If you are new to spinning or if you are fairly experienced, I would wager that you can learn something new from this book. The photos are wonderfully descriptive and she perfectly describes (at least for my learning style) a variety of techniques.
The Fleece & Fiber Sourcebook by Deborah Robson and Carol Ekarius: If you love fiber, this is such a fun book to just pick up and flip through. There are so many amazing fiber options out there and this book describes many of them in detail. I especially love how they provide great photos of the raw fiber, spun fiber, and knitted yarn to give you an idea of the texture.
Hand Dyeing Yarn and Fleece by Gail Callahan: Much of what I learned about dyeing is from this book. It provides detailed instructions on so many different techniques and the chapters are so well written, this woman knows what she is talking about.
The Principles of Knitting by June Hemmons Hiatt: This is a new addition to my knitting library. This is probably the most comprehensive knitting resource I have ever seen, as soon as I opened this book I wanted to read half of the chapters. The only problem I have with this book is the lack of photos in sections describing complex techniques.
Socks from the Toe Up by Wendy Johnson: This is the only pattern book that I have ever found where I love every single design in it. I learned to do toe-up socks two-at-a-time from this book, her instructions made so much sense to me and the photography is gorgeous.
What are some books that you just had to buy or continue to keep coming back to?