The narrative hook - also known as a literary hook - is the literary device whereby you hook the reader's attention and intrigue her enough so that she'll keep reading.
When a reader (whether a beta reader, or a literary agent or publisher, or hopefully a real-life genuine member-of-the-public reader) - picks up your book, you literally have only seconds to impress him or her.
There is so much competition for that reader's attention - whether it's the slushpile, (in the case of agent or publisher) or all the other books in the bookshop (in the case of the end-use reader).
And so, he or she will only allocate a very short period of time - maybe as few as thirty seconds! - to deciding if your work is worth reading.
Therefore, your job as the writer is to make absolutely sure that the answer to that question (“Is it worth my while reading on?”) is an absolute categorical 'yes!' - and the narrative hook is one way to do that.
The most popular way to use a narrative hook - and the easiest to carry off too - is to pose a question in your reader's mind, so that she just has to read on to find out the answer to that question.
There are two options here. The first is to pose the dramatic question early (e.g. Will the hero save the world?). The reader will have to read the whole book to find out the answer to that one!
The other option is to pose a smaller question (examples are given below, in the list of opening lines of famous novels). And, of course, by the time you've provided the answer to that question, you'll already have posed another question to keep her reading to find out that answer, and so on. As I often say: a writer's first job is to keep her readers reading!
Popular advice is to begin in medias res, which literally means in the middle of things. This surely has the advantage of intriguing and hooking the reader. It's not without its difficulties though, as whatever happened to get the characters in that situation then becomes back-story with all the challenges that entails.
Ideally the narrative hook should be in the very first paragraph (even in the first sentence if possible), and indeed I'd go as far as to say that you would be well-served by devoting some time to thinking up a good, intriguing, hooking first paragraph.
The question posed in the first paragraph doesn't have to be answered immediately, however. That hook will buy you some credit with the reader, so that she'll read on for a while in order to get the answer she wanted. (And indeed, that space between posing the question and answering it is where you pose the next question as explained above.)
Although arousing your reader's curiosity is the most popular literary hook, if the quality of writing is good enough that can act as its own literary hook. Likewise, an engaging character or an intriguing setting can pull the reader in. But these are harder to carry off - you might be better off posing the question as to why that engaging character has found himself in that intriguing setting, and cover all bases.
Let me share with you some first lines of famous novels, and explore how those first lines act as a good narrative hook. As you read them, particularly those of novels you mighn't have read, be aware of how you feel. Do you want to read more? I bet you do!
- Call me Ishmael. (Moby Dick, by Hermann Melville)
I really like this first sentence. It immediately begs the question: Why should we call you Ishamel? Is that your name? If so, why not just say 'My name's Ishmael'? And if not, why would we call you it?
- It was a bright cold day in April, and the clocks were striking thirteen. (1984, by George Orwell)
The thing that leaps out of this sentence is, of course, the fact that the clock was striking thirteen. Clocks don't strike thirteen. So why did they in this instance? That's the question which forms the narrative hook here.
- Someone must have slandered Josef K., for one morning, without having done anything truly wrong, he was arrested. (The Trial, by Franz Kafka).
Again the questions: Who slandered him? Why? And what happened after he was arrested?
- It was a wrong number that started it, the telephone ringing three times in the dead of night, and the voice on the other end asking for someone he was not. (City of Glass by Paul Auster)
Again, we just need to know what happened next. What's the 'it' that the call started?
Do you see how these examples have worked so well in order to act as a narrative hook? They hook the reader in, just like a fish being caught. Aim for this with your own work, and it'll give you a huge advantage over writers who don't.