I think the set you are trying to identify is the Illustrated Bobley World Encyclopedia. They had creme colored covers with dark green and gold spines. Also had The Library of Literary Treasures in the back of each volume. Some 60 pages extra. First published in 1967. Set had 21 volumes with a 2 volume World Atlas. Bobley had bought the rights to The Illustrated Home Library Encyclopedia. (My all time favorite.) Slighty edited and updated, they produced the Bobley Encyclopedia. The Literary Treasures was their idea and worked well with their encyclopedia set.
I have a set of these from the ‘30’s or ‘40’s, I think. They are kind of amazing. They have lots of interesting things! They were sold as a serial/subscription, $40 every two months I think. Each volume had a little of everything in it. Geo, history, art, botany, culture, etc, including things to occupy children. Building kites and box cameras, etc. the photos are quite good, the maps a delight. I still enjoy going through them!
It’s amazing how much information can be gleaned from these old encyclopedia sets. my favorite set contains over 5,000 pages not counting the atlas and study guide/index. I plan to skim read the entire set. History doesn’t change much. Technology does, but I can get an update of that on Wikipedia.It makes me feel like a kid again! it’s a good feeling!