As a subject, “either” is singular. It’s the opposite of “both,” and refers to one at a time: “Either ketchup or mustard is good on a hot dog.” But if “either” is modifying a subject in an “either . . . or” phrase, then the number of the verb is determined by the number of the second noun: “Either the puppy or the twins seem to need my attention every other minute.”
BUY THE BOOK!