The reading ease algorithm we use is the Flesh-Kincaid algorithm, which works pretty well for regular prose books but clearly fails very badly on avant-garde prose like Ulysses or As I Lay Dying.

Little Women is 0.07 more difficult than Ulysses.

https://standardebooks.org/ebooks/louisa-may-alcott/little-w...

The "very difficult" books are mostly political theory, including the famously challenging Communist Manifesto at 38.05.

https://standardebooks.org/ebooks/karl-marx_friedrich-engels...