Look in the index for your main keyword(s). Indexes are at the back of the book; there could be a variety of indexes such as Author, Title, Subject/Keyword so pay attention to which section you're in.
Not all books have indexes. In those cases you'll want to read through the Chapter headings listed in the Table of Contents (front of the book).
Follow the "leads" when reading the text -- are other books or articles mentioned? If so, use those titles and/or authors to find more information.
Does the book have sources for each chapter at the end of the chapter or book? (i.e., a List of References, Bibliography, or Works Cited). If so, take advantage and use them to search by the authors and/or titles.
Remember you do not need to use the entire book. Often you'll find a chapter or a section in a book that supports your research.