I received many responses to my query about which type of encyclopedia to buy for a new library. World Book was by far the most respected encyclopedia for a variety of reasons and was endorsed by all respondents. I will post only a few responses here. I wouldn't do without a relatively current set of World Book.
Graphics are great for kids needing pictures of stuff for their own interest or for projects.
If you buy nothing else, buy a set of World Book. They are written with younger students in mind. But I also like the Compton's. I can use it to help students learn how paragraphs are organized with the first sentence being the main idea. The outlines at the back of longer articles, the volumes being divided by the alphabet and so many other features make it the best as far as I'm concerned.