Some old and hard-to-find papers

• Bray, Crawford, and Proschan (1967)
  
  Maximum likelihood estimation of a U-shaped failure rate function.
  Mathematical Note 534, Mathematics Research Laboratory, Boeing Scientific Research Laboratories,
  Seattle, Washington
   
• Saunders (1975)
  
  Birnbaum's Contributions to Reliability Theory.
  In Reliability and Fault Tree Analysis;
  Theoretical and Applied Aspects of System Reliability and Safety Assessment.
  SIAM, Philadelphia, 1975. pages xv-xxxix.