Knowledge is power, and when I was getting my mind wrapped around UX, I would've been lost without these guiding posts to help me along the way. In no particular order, these are, in my opinion, critical design books that help form a solid foundation of knowledge that every UX Designer should read:
The Elements of User Experience by Jesse James Garrett
About Face 3: The Essentials of Interaction Design by Alan Cooper
Universal Principles of Design by William Lidwell
Universal Methods of Design by Bruce Hanington
Communicating Design by Dan M. Brown
The Design of Everyday Things by Don Norman
Don't Make Me Think by Steve Krug