I see "stack of books with legs" Lt. Kirk as an artifact from the pilot. After all, his supposed BFF Gary Mitchell never gets a mention again. Spock wears yellow and has a vaguely-defined human ancestor, no McCoy or Uhura, Sulu wears blue.... the characters weren't defined yet, the costumes and sets were prototypical etc
(oh yeah, and books are obsolete! Everyone bar Kirk's lawyer uses PADDs)
I say this new James Kirk is working back from the Kirk seen in the series proper.