You may want to consider whether you actually want those fragments to be facsimile-like, or simply have some differential formatting that helps the reader know what's happening.
For a handwritten letter, for instance, I, as a reader, do not generally want to see a reproduction of the original (real or fictional), but a properly typeset version. In most books, this is enough (for me):
Code:
blockquote {
margin: 1em 2em;
}
and that applies for the occasional web/online extracts, newspapers and letters.
If a book is a collection of letters, I may remove the side margins and change the font size instead, and special cases may have to be considered if there are letters inside letters or something like that.