There is only one file type that uses the .KCB extension:
Kindle Create Book File
A KCB file is a project file created by Amazon Kindle Create, a program used to create professional-quality books to be published on the Kindle store. It contains information about a book, which includes the book content, table of contents, and book formatting information. KCB files are used to format books to be published on the Kindle store through the Kindle Direct Publishing platform.
|Amazon Kindle Create||Free|
Updated: May 15, 2019