Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
sincerely, faithfully, truly (advs.)
The complimentary close of a letter often uses sincerely in certain formulaic ways: sincerely yours, very sincerely yours, yours sincerely, and sincerely. Americans frequently use truly in place of sincerely in all but the single word closing formula. Some Britons, and a few Americans too, may also use faithfully in some of these formulas. The tone or style of these is mostly a matter of convention, but the single-word versions are probably the most informal, and the three-word models the most overtly formal.
