A blurb (also called a book description or back cover description) is a summary of your plot aimed at readers. It is used on online book pages or on the back of your physical books to tell readers what the book is about.
In my personal opinion, writing the blurb is sometimes the worst part of the book production process. At least once, I have literally had to spend more time writing those few paragraphs than I did editing the entire 100,000 word manuscript that the summary was about. (Granted, I didn't have to write the 100,000 words first.)
It is a delicate process to make all the important details of a full-length novel clear in a few paragraphs and create a sense of the drama and stakes that your characters face. Concision is key, whether your limit is the space available on the back cover, the number of characters allowed on an Amazon page, or just the attention span of your listener. Balanced with that, there is the need to be compelling. The book needs to sound fun, insightful, or useful.
Here are some of the tricks that I have learned over the years both from writing blurbs myself and from being on the receiving end of other people's summaries:
Provide the Essential Information
Readers generally look at a blurb after being pulled in by the cover and title of the book. This means that you don't have to start completely from scratch. They should already have a general idea of the topic, genre, and age range. In your blurb, you want to further narrow that down by clarifying the tone and the plot.
The easiest way to get the basics of your book across is to include the following details:
Protagonist
Readers will likely want to know the protagonist's general age, important traits, and about their genre-related skills. Are they a thief, a magician, a princess? Are they shy or sassy? Are they relatable or awe-inspiring?
Primary goal of the protagonist
This can sometimes be hard to explain without spoilers. But in the most general terms, what type of goal is the book going to be about? Does your protagonist want to fall in love, defeat the bad guy, or save their loved ones?
Antagonist/Challenges to the Goal
Again, this is a place where spoilers can be an issue. Don't worry about explaining everything. But do try to give readers a sense of the type of antagonist to expect. Are the protagonist and antagonist racing to achieve the same goal? Is the antagonist already powerful and needs to be stopped? Are they fighting out in the open or in secret? These type of details will show readers the flavor of your book without needing to know all the details.
Stakes
To help readers understand why they want to read your book, don't make your stakes about saving the world. Even if that is the goal, it's vague and unhelpful to tell readers that. Instead, be specific. Is the protagonist trying to protect a specific person? Are they trying to save the world from a very specific form of destruction? The more specific you can be, the more readers will want to start reading your first page.
I recommend starting a blurb draft by roughly listing these things out. Don't try to make complete sentences yet. Instead, use this as a brainstorming session. Write down lots of details. Word things in a few different ways. List nouns and adjectives you can work into complete thoughts later. Bullet points are a great way to get at the heart of these important details with as few words as possible, something you will be grateful for later.
Think about Style and Tone
The point of a blurb is to help readers decide if they are part of your audience. Aside from your premise and plotline, your themes and writing style will also be a factor. Make sure to use words and phrases that match with the style inside your book. If your book is silly and zany, don't describe the plot in a matter-of-fact way. If your book touches on dark themes, make sure to hint at that in the blurb.
Use adjectives and details that create a mood and atmosphere that will appeal to your audience. Remove or reword any details that don't fit with your style and tone.
Sometimes a quote from the book will convey this better than a summary of the plot. I once worked on a blurb for a Pride and Prejudice retelling that was set in the modern day. The author of this retelling frequently used Jane Austen references with modern day twists, such as "It is a truth universally acknowledged that a boy in high school must be in want of a girlfriend." This tone was a huge part of the reason that people wanted to read this book. Instead of trying to summarize the plot in a series of twisted Jane Austen quotes, the publisher included a quote from the book in the blurb before the summary of what the book was actually about.
Using a quote isn't the right approach for every book, but if your tone is particularly whimsical or wordy, you may find a quote useful to convey that tone while still keeping your summary of your book concise.
Consider your Hook
The point of a blurb is to make readers want to pick up your book, so you really want to include not only a good picture of what the book will be about, but an intriguing hook. What about this book will most appeal to your specific, intended audience?
Most good hooks will fall under the protagonist, goal, conflict, and/or stakes already, but not always. In these cases, you want to hint at your hook through your word choice and chosen details.
For example, if your book has heavy Chinese influences, that may show up more in the setting than in the plot goals. To help show readers this more abstract appeal, you could mention that the inciting incident takes place at a Chinese festival, you could include your protagonist's Chinese name, or you could mention that the protagonist fights the antagonist by teaming up with a Chinese mythological creature. If your Chinese-influenced book is a fantasy story, you can make a point to avoid saying things that would hint at a European setting, such as knights and castles. Ideally, the book cover would reflect this visually as well.
Whatever your hook is, make sure that it is showing up in your blurb.
Drafting
Now that you have a list of the coolest and most important parts of your book, it's time to turn those details into sentences.
The first attempt may end up two pages long. That’s fine. It's often easier to trim down a long blurb than to clarify a disjointed summary. Even if your first attempt gets unwieldy, I recommend that you keep writing it out until you've connected each of the elements from your list.
Be careful about the number of turning points in your blurb. Having multiple turning points likely means that you have drifted out of a blurb and into a synopsis. Synopses are a great tool, but they tend to work best inside the industry. They aren't good for intriguing readers.
Trimming it Down
After you get everything important onto the page, it's time to be concise. Eventually you will want to be able to summarize your entire plot in about 2-4 paragraphs of text and less than 200 words.
The right blurb for your book should show off what your book is about and why readers want to pick it up. This may mean breaking "the rules" a little bit to best highlight what is awesome about your book. As long as the blurb is clear, concise, and compelling, the format of this type of summary is less important than its effect on a reader.
Final Checks
Ask an outside reader to review your blurb. Unlike beta readers, this doesn't need to be someone who likes your genre. Just someone who can read English and hasn't read your book. As someone familiar with your book, you will understand things with much less context than your readers will need, so an outside perspective is super important. Use this outside reader to make sure your blurb is pleasant to read and not boring or confusing.
When paired together, your title, cover, and blurb will be your book's most basic marketing tools. These three things will appear together almost everywhere readers encounter your book. Together they should paint a clear picture of your book and show readers that your book will be professional and enjoyable. If you can accomplish those things, your book will be much easier to get into the hands of the right readers.
Good luck summarizing!