If you've read any other Dickinson poems, you may be familiar with the mysterious nature of the speaker. We don't get any hints about who or what is telling us about books "There is no Frigate like a Book." There are no clues as to the speaker's gender, age, or characteristics (not even an "I" – so we can't even say exactly say that it's a person). The only thing that makes the speaker seem human at all is the pronoun "us" in line 2, which implies that he/she/it is a reader, just like we are. It's perhaps best to think about the speaker here as a kind of disembodied voice, making observations about the natural, human joys of reading.