Letter-perfect means right in every small point — not “quite good,” but exact. If a student is letter-perfect in a poem, every word and every comma is in the right place. If a report is letter-perfect, the facts and the wording have been checked until there is nothing left to fix. The letter here is a letter of the alphabet, the smallest piece of writing, not a letter you post. The phrase is more American. British English often says word-perfect when someone can recite a text with no mistakes. Letter-perfect can go a little wider than speech: it can praise any performance or copy that is exact. It is complimentary, a little old-fashioned in some rooms, still common in American school and theatre talk.