The World Book Encyclopedia was, and is, the most popular encyclopedia for the general audience, at least in The United States of America. From its inception in 1917, it was designed to be more accessible than most encyclopedias, and by the time of my childhood, it had fulfilled this destiny by being the standard encyclopedia in most schools, written in a style understood by a middle school student. It had bright colors, lots of pictures and maps, and easy to follow section headings in major articles. Each volume covered (more or less) one letter, avoiding the situation with, say, the Encyclopedia Brittanica where you would have a volume entitled Sonar to Tax Law.
The World Book Encyclopedia focused on the United States, with each President of the United States (even the boring and unimportant ones), and each State (even the boring and unimportant ones), having a big entry with lots of pictures and diagrams. The encyclopedia also started large entries out on their own page, using padding to ensure that important entries didn't start in the middle of the page.
The World Book was middle-of-the-road, limited in scope, ethnocentric, and sometimes emphasized style over substance, but I certainly learned a lot from it. Also, it had the cool human body overlay transparencies.