besides vs either

besides

adv
  • Also; in addition. 

  • Used to emphasize an additional point, especially an important or stronger reason; moreover; furthermore. 

  • Otherwise; else. 

prep
  • In addition to. 

  • Other than; except for; instead of. 

  • Beside, next to. 

either

adv
  • As well. 

pron
  • One or the other of two people or things. 

det
  • Any one (of two). 

  • Each of two; both. 

conj
  • Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by “or”. 

How often have the words besides and either occurred in a corpus of books? (source: Google Ngram Viewer )