From Popular Mechanics from 1910 comes this advertisement for Nelson's Perpetual Loose-Leaf Encyclopaedia.
Reviewed by the New York Times in 1908, the set was supposed to be
"A book that never grows old, that is, never antiquated, that will give answer years after its publication to the most modern of queries — such a book, one imagines, may be found in the great classic of poetry whose verse, metaphorically speaking, breathes the spirit of perpetual youth." Nelson's claimed it had a permanent editorial staff who were "constantly on watch for all important new facts for the benefit of Nelson's subscribers"
Loose leaf was big business in the early part of the 20th Century. Companies were offering "a loose-leaf system for every purpose." One loose-leaf company began in New York City in 1908 and still makes at-a-glance calendars to this day. Other loose-leaf titles flourished such as Nelson new loose-leaf medicine, Winston's cumulative loose-leaf encyclopedia (read online) and Oxford loose-leaf surgery (read online)
Nelson's was still going strong in 1930 where a set cost $99.50 plus $6/year for updates — buy a set, get a free bookcase — Nelson's stopped publishing updates sometime in the 1930s. Thomas Nelson & Sons is still around today, the world's largest Christian publisher, but their company history curiously makes no mention of their innovative encyclopaedia.
See also: "A Solution to the Problem of Updating Encyclopedias" by Eric M. Hammer and Edward N. Zalta, 1997.