Indexes and Compendiums are indispensable tools for helping us find information! Let me explain why.
The wonderful thing about a Compendium is that it brings together many different subjects, topics, ideas, and philosophies into one neat, tidy volume or volumes. This makes researching or report-writing much faster and easier. A set of encyclopedias is a compendium of thousands of subjects all arranged in alphabetical order. While not as complete or detailed as individual books on specific subjects would be, compendiums give a summary of the most important facts about a subject.
An index is any system that is used to help make finding information easier. Usually found at the end of a book, an index can be a separate volume as well. It can be arranged alphabetically, chronologically, by page number or chapter, by author, by title, etc. The possibilities are endless. And, combined with a compendium, an index can help you find any piece of information you desire.
I've gone to a set of encyclopedias or atlases and used the index, found either in the reference book itself or in the last volume, to find specific articles, maps, or charts. I've even been able to use the set of encyclopedias exclusively to do research, write speeches, and prepare lessons for my class. Everything is right there at my fingertips! Should I happen to need further reading material to create more advanced papers, reports, compositions, or essays, all I have to do is read to the end of each article for further references. At school, for my 4th grade students' primary reference source, the enclyclopedias are perfect!
Indexes are so important in books that I spend some time each year instructing my students to use them properly and effectively, because a tool doesn't do us much good if we don't learn how to use it! So, not only do the students benefit from finding the information they need, they benefit in learning HOW to find it. To use an old cliche, it's like "killing two birds with one stone."