The book description is the main text displayed on a book’s listing page. Here is where you will see the book description on the Amazon listing page:

A screenshot from Amazon.ca, displaying the sales page for the FBCW's book, 'Roots to Branches Literary Contest Anthology'. The book's description is highlighted in green.

Since KDP only has a limited visible space for the book description, it is a good idea to draft the description in your usual word processor before copying and pasting it into the form. You may need to edit the formatting once it has been pasted into the Amazon form. 

A screenshot from the KDP publishing process. The heading is 'Description' and there is a space to input and format a book description. It shows a total character limit of 40000.

Book descriptions on Amazon can be seen as a combination of blurb, sales-pitch and SEO opportunity. Your main goal is to entice the reader to purchase your book, but descriptions can be optimised to improve your rank in Amazon’s search. 

Tips for writing an effective book description:

The first sentence is the most important
If you get the first sentence right, people will read on. You’ll want to introduce the key elements of your book while conveying tone and genre.

Include keywords
This will help improve your search ranking. Good keywords to focus on in a book description include:
– genre, subgenre or tropes
– major themes
– awards
– related books or series (‘… for fans of ‘well-known comp titles‘). Ensure you only choose titles that are comparable to your story to avoid misleading and disappointing readers.

Be mindful of what will fit into the description box without clicking Read More.

A screenshot from Amazon.ca, displaying the sales page for the FBCW's book, 'Roots to Branches Literary Contest Anthology'. The 'Read More' text at the bottom of the description is highlighted in green.

This is what people will see without any extra clicks, so it should be the most impactful part of your description. You have approximately 250 characters to work with (the actual amount can vary).

Get viewers excited
Don’t try and describe every single plot point your story covers. Figure out what parts of your book will excite your readers, and focus on those. Try and use adjectives that convey tone and mood: ‘Empowering’, ‘Thrilling’, ‘Melancholic’, or ‘Hopeful’, for example.

Format your book description – but keep it simple
Use the tools included in the text editor to format your description.

A screenshot from the KDP publishing process. The heading is 'Description' and there is a space to input and format a book description. It shows a total character limit of 40000. The formatting options bar is highlighted in green.

As a rule, make your first two to four lines stand out (bolding or using a heading works well). Quotes (from your story or from a testimonial) can be italicized. Other than that, try to keep the formatting simple.
Unfortunately, there is not currently a way of previewing what your description will look like before publication.
Once your book has been published, review your description on multiple devices (desktop, tablet, phone, etc) to ensure it looks as you intended on a variety of screen sizes.

Proofread
Ensure your description is polished and professional.

More information about Book Descriptions can be found here: https://kdp.amazon.com/en_US/help/topic/G201189630#html


Sources:

How to Write an Amazon Book Description that Sells, by Martin Cavannagh
How to Write a High-Converting Amazon Book Description, by Charles Levin