To me, the most challenging pieces to learn on the organ are modern ones. I am currently working on Acclimations by Jean Langlais. I would like to offer some tips on how to learn a piece like that. The first thing to understand is that many modern pieces are visually complex. There are simply a lot of notes in a measure and this will always make a modern piece hard to learn. A good way to deal with this in practice is to break everything down. First practice your right hand alone, then your left hand. Once you have this down and you begin to form muscle memory, start combining your left and right hands. Once you add the pedal, everything will become much more complicated so you should only add the pedal once you’re able to not worry about the notes in your hands. When you have all the notes down at a slow tempo, begin to increase the tempo in small increments. From then on, you should learn the piece like you would any other, and make sure to correct mistakes before they become a part of your muscle memory.