Year books were printed to be an adjunct to a normal set of encyclopedias. They covered specific events for a year and were a way to add information to the normal set. New scientific breakthroughs etc. They were a way to help keep a set updated with information is all. Not sure when they quit printing encyclopedias, but just find the most recent set you can afford, preferably before all the politically correct shit started (I would say the 1990s) and you will be well served. Don't bother with the yearbooks unless you are looking for a keepsake/gift for someone born that year.