also vs either

also

adv
  • In addition; besides; as well; further; too. 

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 also and either occurred in a corpus of books? (source: Google Ngram Viewer )