(ger)His name is generally written as S. NorthUp. Northrup or Northop are common misspellings, but they are not correct. See also: http://articles.latimes.com/2014/mar/04/nation/la-na-nn-solomon-northup-new-york-times-correction-20140304
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Solomon Northup is currently considered a "single author." If one or more works are by a distinct, homonymous authors, go ahead and split the author.
Solomon Northup is composed of 4 names. You can examine and separate out names.