The Copyright Office was established in the Library of Congress in 1870. It is the official registrar of copyrights in the U.S. Its mission is "to promote creativity by administering and sustaining an effective national copyright system."
Government Documents has a partial collection of Copyright Office publications.
Catalog of copyright entries (LC 3.6/6:)
Annual report of the Register of Copyrights for the fiscal year ending ... (GOVD Fiche LC 3.1:)
We also have many handbooks, guides, and general publications about copyright law, registering copyrights, and searching for them.