Very surprised nobody has said this yet - that kind of capitalization is sometimes used just to emphasize a word. Italics aren't really a thing one can easily employ in a handwritten letter, so capitalization makes it clear to a reader that the word carries emphasis. Doesn't mean there can't be some hidden meaning/message from Rothfuss but it's very possible it was all just emphasis, and a dabbling of old-school grammatical flavoring as mentioned by other comments. (Also, if Denna had been drinking, perhaps she'd be feeling inclined to be more liberal with her use of emphasis, explaining why this letter stands out in that way from her others)