All of these books have less than 50,000 ratings on Goodreads, with many of them only having a few thousand, or even a few hundred, which, when compared to the millions of ratings some of the most popular classics have, doesn't seem like many at all. Also, a few may not be considered "classics/modern classics," but I included them if the author has another book which is considered one - eg. Some of he Margaret Atwood novels may not be considered as modern classics by everyone, but The Handmaid's Tale is, so I felt a few of her other books fit in with this list.
If anyone has read more than 20% of this list then I will be impressed (I most certainly haven't).