The poem is a short, strange love letter to books. Its speaker offers a comparison that seems to come from "the rational tongue," meaning the plain, logical way of speaking: books are like people born in the present, new arrivals who step into the world as lightly as a dancing master. The image suggests that books, however old their words may be, arrive fresh each time someone reads them, and that they move gracefully rather than heavily.
The last two lines make a bolder claim: books make better music than "the most dangerous seas." The sea is loud, powerful and threatening, yet the speaker says a quiet book can outdo it. The feeling here is affection and quiet confidence, with a hint of playfulness in the odd comparisons. The mood is light, airy and a little mysterious, as if the poem wants us to sense the magic of reading rather than have it fully explained.