Hello again, and thanks for your reply.
No apologies necessary. : )
If your work is already published and registered in the Library of Congress, yes, you should be protected, but copyrighting your work is always in your best interest. You can read more about this, here:
"What is the difference between mandatory deposit (in the Library of Congress) and copyright registration?
Mandatory deposit (17 U.S.C. section 407) requires the owner of copyright or the exclusive right of distribution to deposit in the Copyright Office for the use of the Library of Congress two complete copies of the best edition within 3 months after a work is published. Section 408 of the copyright law, for a fee, provides the option to formally register the work with the U.S. Copyright Office. This registration process provides a legal record of copyright ownership as well as additional legal benefits in cases of infringement. Optional registration fulfills mandatory deposit requirements."
where I think you'll understand more about the copyright process, by reading their FAQs. They say that once your original work is published, it is 'automatically' copyrighted, so the librarian was correct, but it's still recommended to copyright, to protect yourself further. If someone used your work without your permission, you would most likely have a legal basis to sue them, but I'm not a lawyer, so I couldn't say for sure. The copyright, in addition to the registration with the Library of Congress, would come in handy in a legal case, if anyone chose to use your work without permission.
I hope this helped you further.