Q:

Earliest use of "rookie"?

What is the earliest use of the noun "rookie" and why has it become the most common synonym for newcomer? What were its vectors? 

A:

A simple search of rookie from etymonline.com

Barrère ["A Dictionary of Slang, Jargon & Cant," 1890] has "Rookey
  (army), a recruit; from the black coat some of them wear," so perhaps
  directly from rook (n.1). Came into general use in American English
  during the Spanish-American War.
The rapid growth of a word from a single seed transplanted in a congenial soil is one of the curiosities of literature [your question of 'vector']
   Take a single
  instance. A few weeks ago there was not one American soldier in a
  thousand who knew there was such a word as "rookey." To-day there are
  few soldiers and ex-soldiers who have not substituted it for "raw
  recruit." ["The Midland Monthly," December 1898]

From the OED

1868 Colburn's United Service Mag.    We..rather disliked the other
  things being done away with, fancying..that they would say it was a
  lot of raw rookies who could not be trusted either to draw a sword or
  spur a horse.

