Copyright stuff.

Basically, you write it, it's yours. You can also REGISTER the copyright.

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?
No. In 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. See Circular 1, Copyright Basics, section “Copyright Registration.”

I haven't followed the case of the purloined letter, but if the newspaper printed it without permission, and copyright resides with the author, then there is a case. The quote also talks about registering in order to sue, but in matters of personal correspondence there are probably features in the law that will come into play.