The Collaborative International Dictionary of English v.0.48:

Letter \Let"ter\ (l[e^]t"t[~e]r), v. t. [imp. & p. p. Lettered
   (-t[~e]rd); p. pr. & vb. n. Lettering.]
   To impress with letters; to mark with letters or words; as, a
   book gilt and lettered.
   [1913 Webster] letter bomb

The Collaborative International Dictionary of English v.0.48:

Lettered \Let"tered\ (l[e^]t"t[~e]rd), a.
   1. Literate; educated; versed in literature. " Are you not
      lettered?" --Shak.
      [1913 Webster]

            The unlettered barbarians willingly accepted the aid
            of the lettered clergy, still chiefly of Roman
            birth, to reduce to writing the institutes of their
            forefathers.                          --Milman.
      [1913 Webster]

   2. Of or pertaining to learning or literature; learned. " A
      lettered education." --Collier.
      [1913 Webster]

   3. Inscribed or stamped with letters. --Addison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lettered
    adj 1: highly educated; having extensive information or
           understanding; "knowing instructors"; "a knowledgeable
           critic"; "a knowledgeable audience" [syn: knowing,
           knowledgeable, learned, lettered, well-educated,
           well-read]