I've been thumbing through books on visual design for a while, and recently, picked up a copy of The Non-Designer's Design Book by Robin Williams.
If there's one book that I'd suggest to a newbie on visual design, it's this one. It's rare among design books in that it offers 4 design principles that are easy to remember, easy to spot when violated, and easy to fix. Over 90% of the slides that I have reviewed violate at least one of these principles (often all), so I guess there's a 90% chance this book will improve your design.
The four principles are (in the order of how often I see them violated):
Contrast and proximity are important for the message. Proximity groups information into messages, and contrast highlights the key message. Alignment and repetition are more important for design. It makes for more appealing reading.
Williams orders these in a different way to create a memorable acronym. (I'll never forget it.)
I'll let you read the book and absorb it better. At less than 200 pages, it's a very readable book.