If you are considering downloading a PDF, remember: respecting copyright ensures that future editions (or similar quality texts) will be written. Use legal library access, buy a used copy, or request an e-book through your institution. The knowledge inside — spacetime curvature, black holes, the expanding universe — is worth the modest investment. Further reading: Ohanian, H. C., & Ruffini, R. (2013). Gravitation and Spacetime (3rd ed.). W. W. Norton. ISBN: 978-0-393-92287-9.