Downloadable in a variety of formats. There are lots of them, so keep clicking on the 'more stories' bar at the bottom. Everything from histories of NASA to technical manuals for designing experiments for the Space Station. https://www.nasa.gov/connect/ebooks/index.html They tend to be very big (up to 100mb) files with lots of color photos. Some have embedded videos for those formats (like epub) that support it.
