One of the advantages of the classification systems used by most libraries — typically the Library of Congress or Dewey decimal classification system — is that they are subject-based. Because books on similar subjects are shelved together, you can browse the stacks to look for sources on a topic. For example, if your research takes you to the stacks for books about alcohol abuse, you’re likely to find books about drug abuse, treatment programs, and codependency nearby. When you find a publication that seems useful, check the works cited list for related works. The combination of browsing the stacks for sources and checking those sources’ works cited lists can lead you to publications relevant to your subject.