Probably the best textbook for this class hasn’t been published yet. And it might be the best because since it’s not yet finished, we can hope any flaws it has will get fixed.
Tamara Munzner has been working on this book for a while. She is making a draft available to us.
Tamara Munzner. Information Visualization: Principles, Methods, and Practice. To be published (draft of 30 Dec 2011). http://www.cs.ubc.ca/~tmm/courses/533-11/book/
You can download a huge PDF there. We have made individual chapters available in the protected course reader.