Complex sentence: The white anthropologist J. Sydney Slotkin, whose book "The Peyote Religion" was written essentially as a manual for the Native American Church, was actually an official of the church and edited its Quarterly Bulletin.
Split into simpler sentences: The white anthropologist J. Sydney Slotkin's book, "The Peyote Religion", was written essentially as a manual for the Native American Church. Slotkin was an official of the church and edited its Quarterly Bulletin.