I was perusing a 
post by the divine Duchesse; she discusses ways to keep your aging mind working. She mentioned a post she wrote a few years ago on reading 
harder books, so I took a look. There was my comment about just having read Louise Erdrich's "Round House."
I had no recollection of reading that book. So I looked at a plot summary on wikipedia. I still have no recollection. That is depressing.
Another reason to keep teaching. I teach "harder books" all the time. And I teach them over and over again. I know them quite well. So well that I could do all the quizzes on the
 Iliad and the 
Odyssey on a great Harvard mooc by classicist 
Gregory Nagy even though I haven't taught those works for many years.
I started teaching a Shakespeare course after the fellow who "owned" the course retired. I felt somewhat rusty at first, but I can now say--after 15 years--that I know the plays quite well.
I just completed the Neapolitan novels by Elena Ferrante. I do remember them (so far).
It turns out that I remember my reading of Proust (took me over a year). That might be because I listen to an audiobook on the way to work.
Two harder books that I have been unable to finish because they are so painful: 
The Radetsky March and 
Austerlitz. I keep returning to them. I can only read a little at a time.
Perhaps re-reading is the key. One Erdrich book I loved and remember quite well is 
The Last Report on the Miracles at Little No Horse.  I read it several times in a short space because I loved it so much.
Ditto for the master of harder books Henry James: 
Wings of the Dove, Portrait of a Lady, The Golden Bowl. The harder the better as far as I'm concerned.  Same for his somewhat less-difficult friend Edith Wharton: 
Age of Innocence and 
House of Mirth.
And how can we forget 
Middlemarch and 
Mill on the Floss? 
Does re-reading help keep one's mind at work? Or is it the book and the reader's mind? I remember some Trollope. But I REALLY remember lots of Dickens.  I read about eight books by Anita Brookner recently but barely remember anything--except a sense of melancholy.
What to read next?