 Finley believes that the abstract index notation is difficult to learn, though convenient for those who have mastered it.Theoretical physicist James W. York wrote that "General Relativity" is a sophisticated yet concise book on the subject that should be appealing to the mathematically inclined, as a high level of rigor is maintained throughout the book.However, he believed the material on linearized gravity is too short, and recommended "Gravitation" by Charles Misner, Kip Thorne, and John Archibald Wheeler, and "Gravitation and Cosmology" by Steven Weinberg as supplements.