It sounds like this letter might have been written by someone who is now no longer living, correct? And possibly the recipient is also no longer living. If either the sender or recipient were still alive, the best thing to do would be to contact either of them requesting permission to post.
If you know who has the original document, you can contact them requesting permission to post at WikiTree. If not, and if the content of the letter is historic, relevant, and already ubiquitously shared online, then it sounds like it will be fine to post a scanned image on WkiTree and also a transcription of relevant portions of the document on WikiTree.
Citing a personal letter as a source is important here at WikiTree and can be done at the end of whatever transcription or summary you include on a WikiTree profile with a format something along the lines of this:
<ref>Joshua C. Barnes, Letter to Mrs. White, 5 Jun 1923. (The original handwritten letter is in the possession of Dr. David Jones, Flushing, New York. Letter ued with permission. Joshua resided in Grosse Pointe, Michigan when he mailed this letter.)</ref>