WordNet (r) 3.0 (2006):

letters
    n 1: the literary culture; "this book shows American letters at
         its best"
    2: scholarly attainment; "he is a man of letters"

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "letters":
   French literature, IPA, ITA, Initial Teaching Alphabet,
   International Phonetic Alphabet, Renaissance literature, account,
   alphabet, alphabetics, ancient literature, annals, belles lettres,
   bibliolatry, bibliomania, bluestockingism, book learning,
   book madness, bookiness, bookishness, booklore, catalog, chronicle,
   classical scholarship, classicism, classics,
   contemporary literature, correspondence, culture, documentation,
   donnishness, erotic literature, erotica, eruditeness, erudition,
   folk literature, futhark, graphemics, history, humane letters,
   humanism, humanistic scholarship, intellectualism, intellectuality,
   inventory, kitsch, learnedness, list, literacy, literature,
   medieval literature, memorial, national literature,
   obscene literature, paleography, pedantism, pedantry, pipe roll,
   polite literature, pop literature, popular literature,
   pornographic literature, pornography, pseudonymous literature,
   reading, record, recording, register, registry, relic, remains,
   republic of letters, roll, rolls, roster, rota, runic alphabet,
   scatological literature, scholarship, script, scroll,
   serious literature, syllabary, table, token, trace,
   travel literature, underground literature, vestige,
   wisdom literature, writing system