The Non-Designer's Design Book by Robin Williams
I am not a natural-born designer. Growing up in my family, I was the writer--the one who was good with words. It was my younger sister who was the artist, and heaven help us should we try to cross over into the other's territory. So no one was more surprised than me when I went into web design (well, maybe my younger sister.)
Without any formal design training and harboring the secret knowledge that I am not the artistic sister, the graphic design aspect of web design is the one that makes me most uncertain and nervous. Clearly, I can't stay in this profession without addressing my lack of knowledge.