Complex sentence: In 1980 HarperCollins published the series in order of chronological of the events in the novels, which meant "The Magician's Nephew" was numbered as first in the series.
Split into simpler sentences: In 1980 HarperCollins published the series ordered by the chronology of the events in the novels. This meant "The Magician's Nephew" was numbered as the first in the series.