If you create digital products, you have a responsibility to make them easy to use. It’s mostly common sense; it’s just not common practice! These books are what I consider the must-haves for any user experience designer. They'll teach you how to design products that are useful, usable and desirable.
The field of user experience design is so encompassing that there is a near infinite amount of knowledge to be drawn from other domains. Whilst UX as a discipline is very young it's founded on principles and research from psychology, design and social sciences that give it a definite foot up when it comes to feeling confident in the work we do.
Books on the tools and processes that have taught me the most about not only how to create interface designs, but how to think about design and the techniques that make that thinking concrete and communicable.