I read two other of Hawking’s books, A Brief History of Time, and Black Holes and Baby Universes many years ago.  At that time, I felt like I understood them reasonably well.  This book was a challenge.  I think I grasped most of it while I was physically reading it, but the concepts are pretty esoteric. Very interesting, though.  He was quite the genius, and it was no easy task explaining these complex theories in lay terms.