Cite a published collection of letters using italics in both Modern Language Association (MLA) and American Psychological Association (APA) styles. Cite the collection like you would any other book in these citation styles. In APA, for example, list the editors of the book, "Eds." in parenthesis, the year of publication in parenthesis, the title of the collection in italics, the location of publication and the publisher to complete a reference page entry for a collection of letters.
Cite a single letter from a collection of letters the way you would an article or work in an anthology in both MLA and APA style. In MLA, this means adding the title of the letter in quotation marks along with the letter's page range to the traditional book citation format, in which the title of the collection would still appear in italics. For example: Smith, John. "Letter 22." The Collected Letters of John Smith (in italics). New York: Letter Press, 2009. 22-24. Print.
Use italics to note the name of the newspaper when citing a letter to the editor in MLA and APA style. In MLA, for example, the Purdue OWL notes that a letter to the editor should be cited "any article in a periodical," but the author must write in the citation that the work being referred to is a letter.
Do not use italics to cite personal communication, like letters or e-mails, in APA or MLA style. Cite unpublished personal communication by listing the writer's name, title (if any), date, to whom the message was sent and the type of message (letter or e-mail, for instance).