If you are having trouble deciding which parts of which books are the ones you should be spending your time on, use the index. Look up several key names or words and see where in the book they appear most often. If, for example, you are writing an essay about Christian responses to Darwin's theory of evolution in the nineteenth century, there will be lots of different things you can look up, based on the names and ideas that you have gleaned from your lectures.
Imagine that the index contains the following entries:
creation, concepts of: 24, 57, 70, 73, 74, 76-7, 92, 127, 136, 141, 143, 146, 161, 162, 168, 175, 197, 210, 220, 232, 236, 237-8, 243, 246, 251, 271-2, 279-80, 281-3, 285, 293-4, 303-4, 305, 308, 311, 313-17.
Darwin, Charles: 4, 14-15, 29, 31, 45, 47, 183, 197-8, 219-220, 221, 223, 224, 227, 242-3, 251, 253, 254, 255-63, 270-82, 292-3, 296-311, 313, 316-7, 324, 335.
evolution; Charles Darwin's theory of: 4, 14-15, 16, 227, 231, 243, 247, 255-63, 274, 275-82, 321, 337-8.
Huxley, Thomas: 5, 31, 36, 279, 281-3, 288-90, 305, 313, 321; debate with Wilberforce: 40-2, 49-50, 274.
Kingsley, Charles: 293-4, 314
Temple, Frederick: 41, 274, 310, 317.
theism; theistic evolution: 283, 303, 308, 311-317.
What you are looking for is clusters of pages, which, ideally, are referred to under more than one of the headings. As a rule of thumb, ignore single page references and look only at references to more than one page. Eliminating all single page references results in the following:
creation, concepts of: 76-7, 237-8, 271-2, 279-80, 281-3, 293-4, 303-4, 313-17.
Darwin, Charles: 14-15, 197-8, 219-220, 242-3, 255-63, 270-82, 292-3, 296-311, 316-7, 324, 335.
evolution; Charles Darwin's theory of: 14-15, 255-63, 275-82, 337-8.
Huxley, Thomas: 281-3, 288-90
debate with Wilberforce: 40-2, 49-50. Kingsley, Charles: 293-4.
theism; theistic evolution: 311-317.
I would then decide, on the basis of the questions that I had in mind as a result of thinking about the essay title, which of these clusters of pages I would look at. The key section, in which the frequency of references is highest, will probably be, in this instance, pp. 270-317.