A dissertation by Elizabeth Kattner-Ulrich at the Frei Universitat Berlin (2008) is titled:
"The Early Life and Works of George Balanchine (1913-1928)."
This thesis is available online and there is a link in Kattner's website "Elizabeth Kattner- Dance Artist".
This is a very well-written and well-documented discussion of Balanchine and his contemporaries. In particular there is a discussion of every piece he was known to choreograph up to and including Apollo (1928).
The chapter headings are:
2) First Encounters at Theater School (1913-1920)
3) From Classical to Avant-garde: Early Soviet Influences
4) The Repertory of the Young Ballet (1921-1924)
5) The Bridge from East to West: The Tour of the Soviet Dancers (1924)
6) Choreographer for the Ballets Russes (1925-1927)
7) Apollo: The Final Transformation (1928)
Also includes appendices: Summary of early works; Correspondence from George Balanchine to Serge Diaghilev, and The challenges of dance reconstruction and restaging.
Well worth reading by anyone interested in Balanchine's early works and influences.