letters vs learning

letters and learning both are nouns.

Word	Noun	Adjective	Verb	Adverb
letters	Yes	No	No	No
learning	Yes	No	No	No

As nouns, learning is a hypernym of letters; that is, learning is a word with a broader meaning than letters:

Other hypernyms of letters include encyclopaedism, encyclopedism, eruditeness, erudition, learnedness, scholarship.

letters (noun)	learning (noun)
the literary culture	the cognitive process of acquiring skill or knowledge
scholarly attainment	profound scholarly knowledge