There might be better or more modern references for Differential Geometry but I enjoyed Barrett O'Neills' book. https://www.elsevier.com/books/semi-riemannian-

There might be better or more modern references for Differential Geometry but I enjoyed Barrett O'Neills' book. https://www.elsevier.com/books/semi-riemannian-geometry-with-applications-to-relativity/oneill/978-0-12-526740-3 https://www.elsevier.com/books/semi-riemannian-geometry-with... Also Misner-Thorne-Wheeler and Wald's book on General Relativity.