I've said two things in the past that came together in my mind today.
1. "This book is so great I read it in a day!"
2. "This book is so great I'm only halfway through!" (This said after working on it for a couple of weeks or longer.)
There's a difference, eh?
On the flip side, the same thing could be said for books you're not enjoying, just maybe a little different.
1. "This book is so terrible I'm just going to race through it."
2. "This book is so terrible it's taking me forever to read."