Not only should the source address your subject, but it should also do so in a thorough and detailed manner.  If your subject is only a minor part of the source, then the source might not be the best available.

Does the source you are using contain information appropriate for your topic?  To make this determination, scan the following:

• Table of Contents: This list can be found at the beginning of a book or magazine.  (In European publications, the table of contents is often found at the back of a book.)  It gives you a general view of the material covered.
  
  

• Index:  Usually lists specific information, i.e., names, places, events, etc.     The index will be found at the back of a book.  Most magazines do not have indexes.  Some independent indexes such as The Reader's Guide to Periodical Literature will help you find periodical articles on a given topic.
  
  

• Preface/ Introduction: Found mostly in books, a preface provides background information about the book and/or author(s).  This information should include something on your topic