The theme of “There is no Frigate like a Book” by Emily Dickenson is the power of the book to conquer the imagination. Metaphors carry this theme throughout the poem. First, books are compared frigates (line 1). Frigates were top-of-the-line war ships, very popular in the centuries before the Victorian era. They came in different models throughout the fifteenth and seventeenth centuries, but the common attribute of frigates is that they included both oars and sails and so could gain great speed in the water, giving them an advantage over enemy ships. This also made them incredibly agile in comparison with other war ships, and tactical imagination was free to rear them to approach enemies in ways uncommon to standard fighting ships. For example, while most ships would fight side to side, frigates could attack at unexpected angles. With the frigate metaphor, Dickinson is saying that books have the power to speedily steer through the minds of readers on their way to conquering their imaginations and may take very surprising, unexpected turns in doing so.
The second metaphor, the comparison of poetry to a courser, is similarly used to discuss the powers of words over the imagination. Coursers were the most common medieval war horses, and they were known for their light weight and nimble hooves. War horses were rarely ridden in the medieval era, except with the express purpose of riding into war. They were brave companions to the knights who went to face either victory or the ultimate defeat, death, and so the faster and freer the beasts could move, the longer the knight was able to avoid the deadly disadvantage of being knocked off and forced to combat on foot. In Dickinson’s poem, these coursers are metaphors for “prancing poetry” (lines 3–4). Poetry is lighter than the frigate, with a prancing, or proud and sprightly, way of interacting with the mind (line 8).
Finally, Dickinson uses the metaphor of the chariot—a Roman war cart drawn by horses—to reinforce this theme. Chariots were beasts in war, but they were also frequently used for triumphal marches after a war was won. Thus, Dickinson is ending her poem by establishing the triumph of literature over the imagination. Books have the power to suspend disbelief, stir up emotions, plant new ideas, and especially whisk the reader away on a journey “without toll," or expense to the reader (line 6). Readers can let themselves be carried away to "traverse" lands where they can face any adventurous battle without fear or worry of their experiences having consequence in the real world (lines 5–6). But that safety is not to imply that the journey is unreal in any regard, though it be fantasy. As the last lines imply, where the mind wanders—though the body may be curled up cozily in a window seat—it carries the human soul with it.