Identify the "hooks" in the letter: what are the significant names, dates, places, events, or people mentioned in the letter? One way to do this is to visualize putting the transcription online, and to imagine which words readers would want to click on to get an explanation. Choosing hooks will help you establish the grounds for formulating your research questions.
Questions for the process narrative:
How did you go about selecting hooks? Did you have trouble identifying hooks? Are your hooks all proper nouns—are there any other hooks beyond these? Is there additional information that readers would like that isn't "hook-able" from the letter?