He believed Hawking and Ellis did a great job summarizing recent developments in the field (as of 1974) and that the intended audience is a doctoral student (or higher) with a strong mathematical background and prior exposure to general relativity. He argued that the core of the books consists of two chapters, Chapter 4 on the significance of space-time curvature and Chapter 6 on causal structure, and that the most interesting application is the penultimate chapter on black holes. He noted that mathematical arguments are at times difficult to follow and suggested Techniques of Differential Topology in Relativity by Roger Penrose for reference.