Typography - the style and layout choices you make with text - is a very important part of visual design.
On Web Typography
A short and practical introduction to typography for the web. Unlike the next book, this one does not cover print typography, so it's a more focused read for a digital designer.
The Elements of Typographic Style
The typography bible. If you care about typography, you need to read this. I can't think of a principle it doesn't cover in a practical way. From memory, it's not even that long.