Complex sentence: Willis Carto was known to be a devotee of the writings of Francis Parker Yockey, who was one of a handful of esoteric writers during the post-World War II era who researched Adolf Hitler.
Split into simpler sentences: Willis Carto was known to be a devotee of the writings of Francis Parker Yockey. Yockey was one of a handful of esoteric writers during the post-World War II era who researched Adolf Hitler.