At least, according to Scientific America.

Considering the source and the fact that wordy 19^th century classics are heavily represented, I’d figured the mind improvements came from the complicated sentence structures, the maze of dependent clauses, that sort of thing.  Instead, the criteria was nothing more than a good story:

As we follow the ups and downs of a carefully crafted story, we build connections within the social and emotional regions of the brain. The result, according to recent research, is a better understanding of other human beings and a deeper empathy for others, leading to improved social skills.

In that case, I’m not sure why these books are any more likely to sharpen the mind than any others.  But still, enjoy, enjoy, read, read.