Usability.gov offers numerous articles and links on usability, as well as downloadable documents you can use for doing testing and writing reports.
However, I would like to point out a few usability glitches in the homepage. While the links in the right sidebar are obvious, in the left pane, it is not. It was not clear to me by merely looking at the heading “Usability Basics” that it is clickable. Another glitch is the list of items under each section. For example, “What is usability?” is written like a navigational link. IMHO, in this case, it is much better to provide the links at the homepage rather than forcing me to click on the section heading first before I can get to the actual text.