The 40th anniversary edition of Codex Seraphinianus
Huhm, the one that came to mind for me is House of Leaves, though I guess it's only 21 years old.  It's (almost) fully legible English, but the pages are all sorts of messed up: Text goes forward, backward, sideways, meanders outside standard directions (including through the page), the footnotes span multiple pages and have their own multiple-page footnotes (all telling multiple storylines in parallel) (also one of the footnotes is editing the book as you read it), there's one section written in a simplistic code...There's some pictures of the pages of this book in these reviews:http://www.cornerfolds.com/2014/08/book-review-house-of-leav...https://stingingfly.org/2016/11/01/house-leaves-mark-z-danie...https://www.horrorbuzz.com/2015/09/13/house-of-leaves/#http://www.thefeedbacksociety.com/television/house-of-leaves...