When you read a good book it is so much more than watching a movie in your head. Yes, that happens: we see and hear the characters and all that is going on in the scenes. However, when you read, you not only are passively watching, but you are EXPERIENCING everything the main character is. By the time you finish reading the book, you have bonded with the characters. You have learned and grown with them. They feel like a new best friend - after all they've shared things with you that they shared with nobody else in the book.
So after going through a good book, we are often not left untouched. Some small part of you learns and grows too. Maybe you are battling some issues that your favorite character was battling. Maybe you're taking notes in case you're ever in such a constant-life-threatening-situation. We often gain new perspectives and learn life lessons. This is where a book is more than just a book. It is often why we see memes like this:
I will be going through that tonight. I'm about 50 pages before the end of the book. Yes. I am stalling. Stalling because I'm in the NO-IT-CAN'T-END-YET stage. Just a few more pages... This could be why I am currently addicted to trilogies. I just don't want to let go of the characters I grow to love.
So no. A book is definitely not just a book. It is fun. It is entertainment. It is growing. It is learning. It is experiencing. It is living. It is loving. And yes, when it ends, it is grieving. Have no fear, though. There's more out there!