Just anywhere you open this book It's wiser than a king. Let's open it and take a look. See! It knows everything: Marbles, markets, measles, mermaid, Milkman, miles, and moon. I'd like to be that wise. I'm afraid I shan't be, though. Not soon!
Picture a young person flipping open a huge encyclopedia and being amazed that every page holds something worth knowing. The speaker says the book is "wiser than a king," and the comparison tells you how much power knowledge holds in a child's mind. Then comes the fun part: the list. "Marbles, markets, measles, mermaid, / Milkman, miles, and moon." These words all start with the same letter, as if the speaker has landed on the M section and is reading entries aloud. What's charming is the mix. Everyday things like milkmen and marbles sit beside a mythical mermaid and the faraway moon, as if the book treats them all as equally real and equally worth explaining.
The mood is bright and playful, with a small wistful note at the end. The speaker wants to be that wise but admits it won't happen soon. It's an honest, slightly funny confession of how big the world feels and how much there is still to learn.