Read:
Vivien Schweitzer, "Unearthly Harmonies, Heard in the Dark"
Georg Friedrich Haas, "Five Theses on Microtonality"
Robert Hasegawa, Parcours de l'oeuvre: Georg Friedrich Haas (English)
Haas in UE Musikblätter 5 (English)
Guy Lelong, Liner notes from Dalbavie's Color, Violin Concerto and Ciacconia
Further Reading:
Robert Hasegawa, Clashing Harmonic Systems in Haas's Blumenstück and in vain
Listen:
Georg Friedrich Haas, in vain (2000), Blumenstück (2000), Limited approximations I (2011)
Scores:
Haa in vain for 24 instruments (2000)
Resources:
Universal Editions's Haas page