The year of publication for reference sources becomes important when you cite sources. Correctly constructed footnotes and bibliographies require this information and, although the information page of a source text will often have a copyright year, that is not necessarily the publication year because the text may be a later edition of an earlier book. An accurate date can be found with a simple search of one of the largest libraries in the world, and it is free.
Navigate with your browser to the Library of Congress website. At the homepage, select the "Researchers" category, which is on the upper left of the page.
On the "Researchers" page, look for the "On the Web" section on the lower right of the page. You will see "LC online catalogs." Click on this heading.
This is the main search page for the Library of Congress. Select the appropriate search form and enter the title of the book for which you are searching. If you have a copy of the book, an alternate way to search would be with the ISBN (International Standard Book Number). You can also search by author or request a specialty search by a Library of Congress librarian.