A hook aka narrative hook, is generally the opening of a story that ‘hooks’ the reader’s attention to keep them reading, or so is the wholesale understanding.
I agree. However, you still need to keep your writing engaging throughout the book to keep hooking the reader.
There is nothing more frustrating when a book draws you in, in the beginning only to wan in the middle. I have read many where the writing falters and I lose interest. Do not allow your book to be one of those.
Tip: Avoid using dialogue in your opening sentence. It’s not the end of your writing world; it merely goes against convention. Many agents/editors appear to have a problem with this, though I have read many a good book that opens with dialogue.
However, if you are a novice, try not to go against the grain.