The Organ Book, written by Olivier Messiaen is a set of seven pieces for Organ which is considered as very difficult (8 and 9). Composed for some Roman Catholic periods, the different pieces are therefore tailored for eventsduring the year such as Easter or Whitsun. These seven different pieces are: - Reprise par intervension - Pièce en trio / For the Trinity Sunday - Les mains de l'abîme / For the penitential periods - Chants d'oiseaux / Forthe Easter Period - Pièce en trio / For the Trinity Sunday - Les Yeux dans les roués / For the Whitsun Sunday Olivier Messiaen (1908-1992) was a French organist and composer passionate about Ornithology and one of the mostimportant composer of his century. Inspired by Japanese music, he had a very special way of composing and his work can be identified by its complexity, its diatonic aspect, its harmony with limited transposition, its colour andits additive rhythms. He composed many works related to ornithology and birdsong, including the 'Bird Catalogue' in 7 volumes and the 'Treatise on rhythm, colour and ornithology' in 7 volumes.