​American Printing History Association: A timeline of printing history

Cranes on Letterpress: A good introduction for the layperson. Of course the letterpress-printed hard copy would be better, but here's the PDF.

Pressing On: If you prefer to watch. A decent documentary on letterpress

Letterpress Commons: Sharing letterpress printing knowledge for the common good. Includes a map of letterpress studios worldwide.

Briar Press: a community of thousands of printers and artists dedicated to the preservation of letterpress