The references are given in order of increasing mathematical sophistication. Reference 1) requires only first-year physics and calculus as prerequisites. Reference 2) is suitable for a student at the fourth-year undergraduate or beginning graduate level. Both 1) and 2) are pedagogical masterpieces. Reference 3) is an adavanced monograph appropriate for senior graduate students and researchers.
1) Exploring Black Holes: Introduction to General Relativity, Edwin F. Taylor and John Archibald Wheeler, Addison Wesley Longman, San Francisco, 2000.
2) Gravity: An Introduction to Einstein's General Relativity, James B. Hartle, Addison Wesley, San Francisco, 2003.
3) The Mathematical Theory of Black Holes, S. Chandrasekhar, Oxford University Press, Oxford, 1983.