Reading Order 
I have a copy of the series from 1982, first published 1980, which orders the books by in-world chronology.  I looked up the publisher "Fontana Lions" and found this site: http://inklingsfocus.com/en_GB/brit_1980.html which states that they were the first to publish the series numbered chronologically.  This would be useful information for the article if we can found a reliable source.-