Q. How do I cite and reference a letter?

One of the purposes of referencing is to enable others, including your tutor, to find the information that you have used in your work. As it would not be possible for others to locate this type of source, you would need to add the letter as an appendix, and because of this can refer the reader to the appendix in the main body of your work, e.g. (see Appendix A).