@hackneymartian i have a longer answer somewhere else, but basically the problem is that you have a base and a number of (random) choices, with proper passwords the base is small (62 for alphanumeric) and the number of choices is large, this gives exponential growth with choices. with book passphrases there is a giant base (basically the number of words in all books combined) but only one really random choice (the starting point of the quote).
also thanks for the compliment ^^