.AZW3

Introduction

AZW3 is the file extension for Kindle Format 8 (KF8), the ebook format Amazon introduced in 2011 for the Kindle. It replaced the MOBI format’s limited HTML with HTML5 and CSS, so books can have richer layouts, embedded fonts and fixed-layout pages. Books bought from Amazon are usually protected with DRM; AZW3 files without DRM are common in calibre libraries and for sideloading.

Amazon has since moved its store to newer formats, but Kindle devices still read AZW3.

Structure

An AZW3 file uses the same Palm database container as MOBI: a 78-byte header with the type BOOK and creator MOBI, a list of records, then record 0 with the PalmDOC and MOBI headers and EXTH metadata. The MOBI header’s version is 8. The text records hold the book’s HTML split into “flows”, with separate index records for the skeleton, fragments and table of contents, and resources such as fonts, CSS and images in their own records. A file made by Amazon’s tools may hold both an older MOBI version and the KF8 version of the book, separated by a boundary record.

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