Since you're working in fiction, Marcia, I would base the sections on actual documents, but I would fictionalize them. By which I mean, take the essence of the letters as inspiration, but put them into the words and style of your character. Even though your character may be based on a real person, by the time she's filtered through your own subconscious she will be different than the actual person on whom she's based. When someone has sent us a letter in real life, unless we ask their permission to use them in our work, I don't think it's polite to do so. If the person has died, that's another matter, of course. Still, you may find, if you simply use them as written, they sound a bit flat on the page. Feel free to change them. Make stuff up! That is, after all, what writers do!