Copyright laws: Can I publish a letter that someone else wrote in my book?

If the letters were written for inclusion in the book, do you have something in writing to that effect? Did the authors acknowledge in writing that the letters would be included in the book? Was any money paid or promised (in the form of royalties and the like) for the letters?

The simplest thing to do is get the authors of the letters to sign a release.

I agree with Ms. Mcfarland-Taylor. You should get something in writing from the authors of the letters acknowledging your right to reproduce and publish the letters in your book (and to continue to reproduce, reformat and republish the letters if your book appears in other forms, such as online, in excerpts, etc.) If any payment or royalty was discussed in exchange for the authors granting you these rights, this information should also be recited in the document.

Not sure what the subject matter or content of the letters is, but you should also give some thought to whether the letters contain any damaging or private information about a third party (i.e., someone other than the authors of the letters). If they do, there could be a libel or invasion of privacy claim from the third party, even if the authors did sign the document mentioned above. If the letters were specifically created for inclusion in your book, the risk is probably low, but you should still consider it.