Most reference books have a table of contents in the beginning, which tells you about the chapters in the book. But the index in the back of the book gets even more specific, telling you exactly which page you should turn to for a given subject.
Using a Book Index
Turn to the very back of the book, where the index lives, and look up the topic you're interested in; topics are listed in alphabetical order. Once you find your topic, the page number next to your topic tells you which page to turn to so you can read about that topic. If the topic comes up more than once in the book, there will be more than one page number next to the topic's listing. Sometimes, indexes will group similar topics together under a single heading. For example, a book on preserving food might list all the fruits you can preserve under the heading "Fruit." So, if you can't find the subject you want, check to see if the subject is classified under a logical heading. Check for synonyms, too. For example, if you don't see an index entry for "hazelnuts," that entry might be listed under the synonym "filberts."
Periodical and Multi-Volume Indexes
Periodicals and multi-volume reference series -- such as the volumes of statutes in a law library -- often include their index in a separate, stand-alone volume. In that instance, the index will tell you not only which page to look at but also which issue or volume to look in.