Credo is a library database containing digital versions of encyclopedias and other reference works. Think of it as an academic Wikipedia, only curated by librarians instead of the general public.
Academic and research libraries typically use the Library of Congress classification system to organize their print materials, which means books on the same subject should be next to or near each other on the shelf. This information can be used to find books in person or via the Library Catalog.
Library of Congress call number ranges:
Library of Congress subject headings:
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These reference materials are available online (click the link under View Online).
These websites are freely available online. No username or password necessary!