Having looked around I would say that Gateway to the Great Books set is definitely under copyright (first edition 1963), particularly the first volume which appears to be all original material. The original publisher Encyclopaedia Britannica is selling some sort of ebook format for encyclopedic prices.
https://edustore.eb.com/products/gat...he-great-books
For those who want to save money here is a site that lists the contents of GTTGB
https://www.thegreatideas.org/gbgateway.html which includes the birth and death dates for authors and I'd expect that most of the works should be possible to find at public domain sites.