Complex sentence: He immersed himself in the study of the "Sefer Yetzirah" ("Book of Creation") and its numerous commentaries, which explain the creation of the world and man as based on letter combinations.
Split into simpler sentences: He immersed himself in the study of "Sefer Yetzirah" ("Book of Creation") and its numerous commentaries. This book explains the creation of the world and man as based on letter combinations.