NASA has made available a collection of e-books in three different file formats (EPUB, MOBI, and PDF files) to accommodate as many electronic reading devices as possible.
Recent titles include:
Celebrating 30 Years of the Space Shuttle Program (2012) edited by William Wallack and George Gonzalez
Rockets and People, Volumes 1 – 4 (2005-2012) by Boris Chertok
NASA’s Contributions to Aerospace, Volumes 1 – 2 (2010) edited by Richard Hallion
Breaking the Mishap Chain: Human Factors Lessons Learned from Aerospace Accidents and Incidents in Research, Flight Test, and Development (2012) by Peter Merlin, Gregg Bendrick and Dwight Holland