Complex sentence: James Brown, who had been assistant head to Berkley, became headmaster in 1969; he wrote the history of the school, "The First Five" (meaning first five headmasters), published in 1988.
Split into simpler sentences: James Brown, who had been assistant head to Berkley, became headmaster in 1969. He wrote the history of the school, "The First Five" (meaning the first five headmasters), published in 1988.