Median novel has some 65k words. Take all (consecutive) quotes of 2 to 24 words, and you have some 1.5m phrases. Take the top 666k books (apparently there've been about 130m titles been published in total, about 5m in the Amazon Kindle store), and you're at about 1e12 phrases, or 40 bits of entropy, or worse than a password with 7 random letters/digits/symbols.

You could probably improve on it considerably by selecting fewer books, and only taking quotes starting at some punctuation mark.

For a naturally throttled attack like here (on the phone) that's fine, but for an offline attack (where the attacker has access to the password hash) that can be cracked within days.