As you get further and further in your design career you might find that the philosophical thoughts start to creep in. These books can help you make sense of them.
The Shape of Design
A short book that is also available online for free. Frank has written a short and surprisingly readable book about what the importance of design and our place as a designer.
Understanding Design: 150 Ways of Looking at Design
A collection of short essays that is unfortunately hard to find - it's been out of print for a while. A whirlwind tour of interesting thoughts about design.