The "15 Pièces sur des thèmes liturgiques" were composed for organ or harmonium, so, if you are going to have a go at these, you need to "figure out what notes you will play and those that you won't," since the organ "version" will be a lot different than the harmonium one.
Albert Paul Alain was born in 1880 in Saint-Germain-en-Laye, and lived and worked there for most of his life. He was organist of l’église de Saint-Germain-en-Laye, a postiton he held until his death in 1971. He was the father of Jehan and Marie Claire.
Attached is the score that is earlier published by "Agnus Dei"