e vs letter of the alphabet

e and letter of the alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
e	Yes	No	No	No
letter of the alphabet	Yes	No	No	No

As nouns, letter of the alphabet is a hypernym of e; that is, letter of the alphabet is a word with a broader meaning than e:

e: the 5th letter of the Roman alphabet
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of e include alphabetic character, letter.

e (noun)	letter of the alphabet (noun)
the 5th letter of the Roman alphabet	the conventional characters of the alphabet used to represent speech
the base of the natural system of logarithms; approximately equal to 2.718282...