The
interpretation of dreams by Sigmund Freud - I am reading it now for the same purpose. Although I don't agree with some of the literal interpretations of dreams that he lays out in that book - I still think that his ideas of 1) the dream as a window into the unconscious, 2) the presence of repressed childhood memories in psychoneuroses, and 3) the unconscious having an influence on our thoughts/actions -- are very interesting. The book (for me, a person who has not read a ton in these recent years) is very challenging to comprehend. Freud has very roundabout arguments, and interpolates all kinds of long, only loosely relevant stories/anecdotes in between his propositions and the ideas supporting them. Many people have a tough time reading his books because of their scientific denseness. You can check a couple pages of goodreads comments/reviews to verify. The lexile score is about 1300, I believe (moderately difficult). I recommend this book.
The
Wealth of Nations is another nonfiction book, specifically known to be a tough read. The lexile rating on this book is ~1500, which means that this book is statistically likely to be a very challenging book. In the real world, many people do not make it very far through this one. It will probably be somewhat boring; at least the reading will be highly educational, in a field that isn't totally obscure (economics). This is the book I will be reading after I complete the former, which will be in about 160 pages.
Novels that have a high lexile rating (which means it is statistically likely to be difficult, as mentioned) include A portrait of the artist as a young man by James Joyce, as well as
A Fable by William Faulkner.
A Fable has a lexile rating of 1500,
Portrait has a rating of about 1200 (if my memory serves correctly). Yes, A Fable is rated to be much more difficult. If you look at ratings by actual humans on sites such as goodreads or Amazon, you will verify that
A Fable is considered a very difficult read.
Other good books to read (from my research):
The Republic by Plato
The Sound and the Fury by Faulkner
On Liberty by John Stuart Mill
Moby Dick by Herman Melville
Various philosophy books
Many classic nonfiction books, a list of the top 100 nonfiction includes
On the Interpretation of Dreams as one of the top books. Check it out:
http://thegreatestbooks.org/nonfiction
I would speculate that reading challenging material will help you more than reading at an easier level.