My grandmother had a set, which I assume were originally owned by her father (who btw knew Frank Hornby) and I have them still. Everything you needed to know in 1906 England!
I have the feeling that though they are huge bound volumes, they were originally published as partworks as you never get a complete treatment of any particular subject, but the breadth of subject covered is tremendous, including languages, various branches of shopkeeping, the different opportunities in the civil service, even building and operating railways... ANd thats what I can remember without popping downstairs to refer to a random volume.
What I do regret is my parents chucking a complete, early, set of Arthur Mees Childrens Encyclopedia. One of the illustrations I recall is that of "Electricity in the Home", which included a maid ironing in the kitchen and children playing with an electric train set, both powered off ceiling light sockets. I don't think the train had a step-down transformer either...