cover letter (letter in a job application where one expounds the reasons why one is interested in the job being advertised)

to the letter

(finance, banking) bill of exchange

to be ignored, not to be taken into account, to go unheeded, to have no effect

bread-and-butter letter, bread-and-butter note

letters patent of nobility / good reputation

(banking) letter of credit / (diplomacy, chiefly in the plural) letter of credence, letters credential

letter of recommendation

to the letter, literally

(informal, simile) to go very well, without a hitch; to hit the spot, to be just the thing