Funny thing is that I am not a designer too! I'm a business/web developer usually focused more on the code's side rather than UX side. This is why I read: the Understanding Color: An Introduction for Designers Thinking with Type: A Primer for Designers: A Critical Guide for Designers, Writers, Editors, & Students and Illustrator CS4 for dummies

Based on your question I would go for the first 2 books: they are useful even for word processing and powerpoint slide-making