Jenkins, Dennis R.,  Published by NASA through the U.S. Government Printing Office, Washington D.C.  128 p. June 2000.  

Before the Space Shuttle, the Air Force and NASA were working on planes that could push the limits of speed.  This document covers  many of those aircraft, including detailed reporting on the X-15 and related aircraft. 
Hypersonics Before the Shuttle (9.9MB)