NASA recently debuted the first in a planned series of free e-books. "X-15: Extending the Frontiers of Flight," by Dennis R. Jenkins is available now in a variety of e-reader formats (the Scott Crossfield Foundation offers a nicely formatted pdf version [direct link] for those wishing to read on computer).
From NASA: "The X-15 was the ultimate 'X' vehicle. Built in the 1950s, she became the fastest and highest-flying winged aircraft of its time. During 199 flights from 1959 through 1968, she collected data about hypersonic flight that was invaluable to aeronautics and to developers of the space shuttle. This book describes the genesis of the program, the design and construction of the aircraft, years of research flights and the experiments that flew aboard them."
A second title, Mark D. Bowles' Apollo of Aeronautics: NASA's Aircraft Energy Efficiency Program, 1973-1987, is slated for release soon.