References for relativity:
<ul>
<li>O'Neill <i>Semi-Riemannian Geometry with applications to relativity </i>
Academic Press 1983
<li> Wald <i> General Relativity</i> Chicagon Univ. Press 1984 
<li> Hawking & Ellis <i> The large scale structure of space-time </i>
    Cambridge Univ. Press 1973 
<li> Misner, Thorne & Wheeler <i> Gravitation</i> Worth Publ. 1973 
<li> Schutz <i>A first course in general relativity</i> 
    Cambridge Univ. Press 1985 
<li> Dirac <i>General Theory of Relativity</i>
  Princeton Univ. Press 1996
</ul>