The speaker is someone with a close and longstanding relationship with books, either a reader or a librarian themself. Although they are offering the reader a cautionary lesson on the dangerous nature of books, they betray a deep understanding of them that can only come from many years of reading. This contrasts the speaker against real-world figures, such as misguided parents and politicians, who may offer express warnings. The speaker is one who has been changed by their experiences with books.