Terje Aven & Uwe Jensen 
Stochastic Models in Reliability [PDF ebook] 

The aim of the present book is to give a comprehensive up-to-date presentation of some of the classical areas of reliability, based on a more advanced probabilistic framework using the modern theory of stochastic processes. This framework allows the analyst to formulate general failure models, establish formulae for computing various performance measures, as well as to determine how to identify optimal replacement policies in complex situations.