It's so hard to pick just 3 books , so I am going to have to cheat a little to answer this, and talk about 3 series every library should have:
1) Amelia Bedelia: the adventures (and comedies of errors that ensue thereafter), of Amelia can be enjoyed at any age. Even as an adult, I can appreciate the literalism (of 'drawing' the curtains or 'changing' the towels), but I also think the clever use of homonyms can be a humorous lesson in English at the elementary school level.
2) Chronicles of Narnia - It's just so imaginitive, and the children are the heroes -- it's inspiring.
3) Tales of Fourth Grade nothing - lessons in family, love and friends