Using a phrase in a book or a poem is not much stronger than using a single word as a passphrase. Attackers include such phrases in their cracking dictionaries, including variations on spelling and punctuation. After the initial hashing of the candidate passphrase, a five-letter word takes exactly as long as a 100-character lyric from a song.

There are lots of sad stories of people losing funds in the early days of Bitcoin when "brain wallets" were briefly popular. Victims used quote-based passphrases that seemed unguessable.

The threat model is a little different when it includes getting access to your encrypted password-manager database or OpenPGP smart card. But the point stands that a well-known phrase might as well be a dictionary word.