In the writing world, a hook can be used to refer to several things, but at its most basic level, a hook is anything that will catch a reader's attention. There are many kinds of hooks, such as an elevator pitch, a strong opening line, or the kind of intrigue that keeps readers interested past page two of your book. This post is focused on the kind of hook that you can (and should) include in a plot summary.
The first step to understanding your story's hook is to separate out what you like about your book from what your readers will like. The reason you wrote the book and the reason people will want to read the book are usually completely different. That’s not bad. It just means you have to do your research. One of the frequent warning signs of an amateur author is marketing a book based on the reasons the author wrote it, rather than thinking of the audience. When I worked for a publishing house, this showed up in a lot of the pitches I received in the form of "this character is based on my niece" or " I wrote this book for my wife who didn't think I'm romantic". These kinds of reasonings show that you are still focused on writing the book and haven't turned your attention to selling the book.
That's not to say that your reason for writing the book is never the same as the hook for your audience. If you wrote a book because your nine year old niece was getting bullied at school, marketing the book to parents of children who are bullied can be a great approach. But not all of your reasons will translate to your audience, so make sure to question them before considering them as your hook.
At a recent writing event I heard Michaelbrent Collings sum this idea up by saying that readers pay you and you pay therapists. There is nothing wrong with writing for yourself, but when you choose to publish a book, you have moved away from looking at what the book offers you. The focus has to move towards what the book offers your audience. After all, they are the ones paying for it. So you have to offer them something that is of value to them.
Your hook should be something that will entertain your audience. At its core, fiction is all about emotion. People read books in order to feel something. That emotion will differ from book to book and genre to genre. But if you understand what emotions your book will create for your reader, you are more likely to be able to promise your audience something that interests them.
It's also important that the hook you offer readers is something that pays off in your book. You may have a fun side character who is an astronaut, but if space travel has nothing to do with your plotline, that's a bad hook.
Look at the major elements of your story. Do you have an interesting protagonist? Is the story set in a fantasy world or historical time period? Do your characters handle conflict in an interesting way, such as scaling tall buildings or reading minds? These things will show up in almost every chapter, and therefore readers who come looking for them will be very satisfied with your book.
To find your hook, you can consider these major elements of your story:
the protagonist's important character traits and genre-related skills.
the antagonist/challenges to the goal
the stakes of your climax
the major genre elements such as the setting or primary method of conflict resolution
the tone and voice of your writing
If you're not a natural marketer and you find it hard to think like your audience, you can get help figuring out what your hook is. If you've had beta readers or a writing group, you can ask them what was the most interesting or intriguing parts of your story. They might name a favorite character, a surprising plot twist, or a specific magical power. They also might tell you something that encompasses the entirety of your story. Perhaps they liked your sense of humor or the Asian influences. Not all of these answers will be your hook, but if you ask enough people you should get a sense of the things that are sticking with your readers and making them feel something. This is a great starting place for identifying your hook.
If your manuscript is still pretty untouched by outside readers, you can tell a couple people about your story aloud. As you cover the major elements of your story, you’ll usually notice a point where people go, “Oh, that’s neat.” Again, this won't always be your hook, but after enough responses you likely will notice patterns. I usually recommend this approach for finding your elevator pitch, but it can work well to find the hook for your longer summaries as well.
Once you have some ideas about possible hooks, make sure to test them for a few important features. Does this hook show up early on in your story? Does it feature prominently in the entire book? Does it pay off in the end?
Don't forget to check the hook against your book's audience specifically. Horror readers may be interested in a story about a werewolf who murders their family, but paranormal romance readers are looking for something else from a werewolf book. Your hook should be aimed at your genre and audience.