This whole poem is kind of a growing pile of transportation-related metaphorical language. The central idea is a simple one: books carry us places – so do boats, horses, roads, and chariots. Ta-da! You've basically figured out this whole poem. However, this simple explanation doesn't do justice to how charming, lighthearted, and fun this poem is. By piling on the transportation images, the speaker gets us to think about different types of journeys we can go on, just as there are all kinds of different books we can read, and different imaginary trips we can take.