There are plenty of graduate textbooks on general relativity, points out Cheng (physics, U. of Missouri), and plenty of popular works describing in for lay readers. He offers an intermediate textbook for undergraduate students that can be used by instructors who are not themselves specialists in relativity. Students are expected to have the usual mathematics at the calculus level, some familiarity with matrices, and the physics of mechanics and electromagnetism including differential equations of Maxwell's theory.
