Image from Dryden Image Gallery/NASA photo
NASA has a new, free ebook available entitled X-15: Extending the Frontiers of Flight by Dennis R. Jenkins.
Here is a short description of the book from the Aeronautics Research Mission Directorate web site:
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.
You can find it here: X-15: Extending the Frontiers of Flight” by Dennis R. Jenkins