A couple of things about the letter. Don't use paragraph spaces. It looks awful. So you'll need indents. I suggest an indent of 1.2em. Next, don't make the letter smaller than the default base text size. That just doesn't work. Being offset is enough. A blockquote works to surround the letter.
Code:
blockquote {
margin-top: 1.2em;
margin-right: 1.5em;
margin-bottom: 1.2em;
margin-left: 1.5em
}
Oh and if the rest of the book has paragraph spaces and no indents, it's going to look awful. You'll want to get rid of the paragraph spaces and add in indents except for the first paragraph of a chapter and the first paragraph of a new section.
If you are going to use an embedded font for the letter(s), then you'll need to specify someplace where it will be seen that publisher font needs to be turned on because there are embedded fonts. If you don't do that, some people won't see your embedded font(s). Kobo & nook Readers need to have publisher font turned on and same with Marvin (for iOS). Kindles are like that as well.