The Holstee Manifesto. Everyone should read this daily.
The most important thing a creative person can learn professionally is where to draw the red line that separates what you are willing to do, and what you are not.
Art suffers the moment other people start paying for it. The more you need the money, the more people will tell you what to do. The less control you will have. The more bullshit you will have to swallow. The less joy it will bring. Know this and plan accordingly.
Neil Gaiman at his address to the 2012 Graduating Class of the UofA in Philadelphia.