The primary reason I will always advocate this book series over the films is the amount of detail Rowling includes to flesh out her characters and her stories. From the minor characters that aren’t even mentioned in the films to the side adventures Harry, Ron and Hermione take somewhat regularly you’re missing out on so much if you’ve never read the books.
And the text books and books Hermione reads! OMG, so many are just throw away lines and titles but what I wouldn’t give to read An Appraisal of Magical Education in Europe or Hogwarts: A History or any of the others that are mentioned! I actually got super excited that she’d released Hogwarts: A History, but it’s actually Harry Potter: A History of Magic (Pottermore link). I guess I can’t really be mad, but fingers crossed it’s next!