Nonfiction by Daniel J. Levitin.
My daughter looked at the cover and size of this book (528 pages!) and said, "Who would want to read THAT?!"
Granted, it's not the sort of thing I usually read, but....
This was a really interesting in-depth look at how the mind works, how we store and retrieve information, and how we can use our knowledge of these two things to make our own lives better. In particular, I liked the chapter about medical decision-making.
I'll admit I didn't read every word of this weighty tome, but it's an amazing book.