If Manfred Eicher considered this behemoth a risk worth taking on double-vinyl, come the 80s he baulked at releasing a much more costly reissue on the new shiny silver discs, and a 40-minute 'highlights' CD was put out instead. It wasn't until 2013 that a double-CD reissue appeared and reviews offering comparisons to Messiaen, Ligeti and pre-synth Klaus Schulze caught the attention of people like me who read album reviews and think less 'oh, that sounds nice' and more 'challenge accepted'.
So if an hour and a half of organ improvisations sounds remotely appealing to you, I wholeheartedly recommend downloading Hymns/Spheres and turning it up extremely loud. If you're looking for an overview of this unique experience, head straight to Spheres (2nd Movement), the longest and IMO best track. Jarrett uses long, droning smears of sound and explores the pre-electric weirdness of the organ stops to full effect (see the sleevenote quoted below), but there's still a melodic logic just under the surface. More abstract and spacy Movements like 3 and 7 gather darker clouds, the album being well sequenced to ensure there's no drag when going the full distance. The 'Hymns' are the bookends to the album, and both are stately, gorgeous pieces of cod-baroque; Spheres (5th Movement) is also a good halfway point that is effectively a third 'Hymn', sharing some melodic material with the finale.
"These improvisations were recorded on the "Trinity Organ", the larger of the two Karl Joseph Riepp (1710–1775) organs at the Benedictine Abbey Ottobeuren.
No overdubs, technical ornamentations or additions were utilized, only the pure sound of the organ in the abbey is heard.
Many of the unique effects, although never before used, were accomplished by pulling certain stops part way, while others remain completely open or closed.
Amazingly, baroque organs have always had this capability."
(uncredited sleevenote, presumably by Jarrett or Eicher)