Books are a long medium. You can’t absorb a book in a single glance. You can't consume it in a few minutes. They take hours, days, weeks, and sometimes months for people to fully enjoy. With a medium that is so long to consume in its entirety, you will constantly find yourself having to explain it to people in a much, much, much shortened version.
There are a lot of times and ways that you will need to summarize your book: pitches, queries, blurbs, online book descriptions, elevator pitches, synopses, and infinite others. Each of these types of descriptions will have slightly different goals. In order to best achieve those goals, you will want to understand the different purposes and rules of these formats. Mixing up a synopsis and a query summary is likely to get your book rejected by an agent. Using an elevator pitch as your book description is likely to leave readers unsure about spending the money to purchase your book.
To help you understand the basic types of descriptions, I've written up a short list. This isn't all-encompassing. Sometimes you will want to mix and match pieces of these descriptions or write a new description aimed at a highly specific audience (such as a podcast host or writing conference committee). But in general, these basic summary formats will help you meet industry expectations and get your message across quickly and clearly.
This is the shortest type of summary. An elevator pitch should be a single sentence. At most, you can fit in two sentences.
In this short amount of space, you will not have time to adequately explain your plotline, characters, magic system, or other unique and important details. Instead, the elevator pitch has to get across the general idea of your book in a way that sounds interesting and exciting. The goal of this summary is to get someone interested quickly, so that they're willing to stick around for more information.
You'll want to focus on abstract details, like the genre, audience, and premise, rather than the concrete plotline. You'll want to avoid any made-up terms or specialized terminology unique to your book/ideas. You'll want to avoid naming your characters, unless they are famous people. You may include a comp title, if it explains your book well, but unlike normal comp titles, one used in your elevator pitch should be a household name.
Here are some examples of what an elevator pitch might look like:
Harry Potter meets Journey to the Center of the Earth.
A young man discovers he is a robot and desperately tries to keep his wife from noticing through ridiculous hijinks.
A synopsis is a summary that's length is measured in pages. These usually refer to single-spaced pages. If the length isn't specified, you can assume the synopsis should be one page, but sometimes an agent or editor will ask for a slightly longer length.
Synopses are not about being quick and pithy. Instead, they are focused on the play-by-play events of the book. All the major plot events are listed in order and the climax is completely spoiled.
I am not as familiar with synopses as I am with blurbs and queries, but here is a great resource if you need to start writing a synopsis for your book.
A blurb, also often called a book description, is the summary of your book that is shared with customers. It will go on the back of the book, the amazon page, and your website.
Blurbs are expected to be a few paragraphs long. They aim to be less than 200 words, though they may be further limited by the available space on the back of your cover or the Amazon character limit.
A blurb should not give away the ending of your book. It should leave an open question that makes readers want to open to the first page and start reading. This means that you should stay away from the play-by-play format of the synopsis and instead focus on the inciting incident, the stakes, and your protagonist's goals.
For more details about how to write a compelling blurb, you can check out this blog post.
The last major type of summary is the query letter summary. A query letter is not entirely made up of a summary of the book, but the summary does make up a large and important portion of a query letter. Like a blurb, you have only a few paragraphs and should focus on the inciting incident, the stakes, and your protagonist's goals.
Instead of focusing on why a reader will enjoy this book, a query summary is focused on convincing an agent or editor that they can successfully market this book to an existing segment of readers. The query summary should clearly indicate that you understand the expectations of your genre and age range, as well as what this agent or editor is looking for.
Query summaries don't have to avoid spoilers as much as blurbs do, because agents and editors are more concerned with proof that you've written a complete plotline than with getting surprised by the ending. Still, they don't want a play-by-play summary. They want to know about the single plotline that encompasses the entire book. To show agents that your story is cohesive, you'll want to have a single turning point (the inciting incident) and tie everything else back to this main goal and conflict.
You can check out these blog posts for more information about writing a query summary or a whole query letter.