A couple of years ago, I purchased the book How to Be a Graphic Designer without Losing Your Soul by Adrian Shaughnessy. Apart from TypoMundus 20, it's probably the second best book purchase I've made.
I originally purchased it for one chapter — the first one — for my Senior Project students to read, but quickly realized the entire book was spot on. Since then, about once a year, I pull it out to read through it. Not only does it include insightful ideas for designers, there are interviews and thoughts from Stefan Sagmeister, Neville Brody, and countless other influential thinkers and doers in design.
Originally intended for design students, it's a must-read for all designers at any stage. You can purchase it on Amazon for a mere $12 plus change.