A few months ago at New Adventures in Nottingham Jon Tan said this:
"Type shapes our experience. It paints pictures that echo in our minds long after we left."
The gist of that quote is pretty much what Type Matters is about. The book, by author Jim Williams, is a collection of examples of how small changes to type and typesetting can create a different feel to how you experience the text.
He explains that typography is an invisible art. Good typesetting will go unnoticed and be a joy to read. Bad set type will irritate and may not deliver its content successfully to the reader.
If you are curious about the mysteries of typography, or just want to learn about it, then Type Matters is a great place to start.