“Dave’s Down-to-Earth Rock Shop” by Stuart J. Murphy did a good job of explaining the concept of classifying in a story format. The rocks are sorted by size, color, type, and hardness. The information about rocks was presented in a simple straightforward manner; enough to make sense, but not too much to be overwhelming.
“Do You Know Dewey?: Exploring the Dewey Decimal System” by Brian P. Cleary was a great introduction to the Dewey Decimal System, using rhyme and fun colorful illustrations. The text itself is quite basic, with further information provided at the end of the book.