Books may provide a comprehensive overview of your topic. Scan the index or the table of contents for key ideas, arguments, and thinkers. Use the bibliography for additional leads.
Much as you might like to, you don't have time to read lengthy monographs from cover to cover. You need to read quickly and efficiently to locate the information you need. This process is an accepted academic practice for assessing a book's relevance to your line of inquiry. Here are some steps to help you analyze a book's relevance.
Once you've skimmed several books this way you'll know which books require closer reading and which can be safely ignored.