In the United States, the best way to register a copyright on any kind of work (including literary works), is through the Library of Congress Copyright Office.
As of June 2018, registration of a manuscript costs $35 and can be done online directly by uploading a copy of your manuscript. Processing times will vary depending on the method used, but range from several months at least.
The Copyright Office’s FAQ provides some helpful information:
When is my work protected?
Your work is under copyright protection the
moment it is created and fixed in a tangible form that it is
perceptible either directly or with the aid of a machine or device.
Do I have to register with your office to be protected?
general, registration is voluntary. Copyright exists from the moment
the work is created. You will have to register, however, if you wish
to bring a lawsuit for infringement of a U.S. work.
Why should I register my work if copyright protection is automatic?
Registration is recommended for a number of reasons. Many choose to
register their works because they wish to have the facts of their
copyright on the public record and have a certificate of registration.
Registered works may be eligible for statutory damages and attorney’s
fees in successful litigation. Finally, if registration occurs within
five years of publication, it is considered prima facie evidence in a
court of law.
I’ve heard about a “poor man’s copyright.” What is it?
The practice of
sending a copy of your own work to yourself is sometimes called a
“poor man’s copyright.” There is no provision in the copyright law
regarding any such type of protection, and it is not a substitute for
Is my copyright good in other countries?
The United States has
copyright relations with most countries throughout the world, and as a
result of these agreements, we honor each other’s citizens’
copyrights. However, the United States does not have such copyright
relationships with every country.
Can foreigners register their works in the United States?
Any work that is protected by U.S. copyright law can be registered.
This includes many works of foreign origin. All works that are
unpublished, regardless of the nationality of the author, are
protected in the United States. Works that are first published in the
United States or in a country with which we have a copyright treaty or
that are created by a citizen or domiciliary of a country with which
we have a copyright treaty are also protected and may therefore be
registered with the U.S. Copyright Office.