Slightly uncanny timing; I was browsing my parents copy (copies?) from the late 1970s over the weekend. Like yours, their acquisition is recent.
They were given a copy as a wedding present in the late 70s.
In the early 90s as we were moving overseas they decided it was too heavy and never used, so got gave it away.
In the mid-90s (pre-internet, but I had a copy of Microsoft's Encarta on CD-ROM for my Mac - which was really rare where we were) they decided to buy a children's encyclopedia - not much smaller than this.
During a move a few years back, having lamented how the internet had taken over encyclopedias and yet they'd brought our childrens encyclopedia back across the world only for it to continue to sit on a bookshelf unloved, they were discussing giving that away.
Last year they bought the above set, 'rebuying' their wedding present. Which now sits under the childrens encyclopedia.
I totally get it though. I've not had time to sit and read for a long time, but while with them over the weekend I started browsing A and before I knew it an hour had passed. In the same way as you can end up going down a hole on Wikipedia with links, but only this time with real paper and actually learning things. Encyclopedias are fantastic things.