AZK is another binary format created by Amazon for reasons that are currently not clear. The Kindle Previewer will convert files to this format if iPhone or iPad is selected. The previewer itself will not read this format. This seems to be a preview test format for KFX.
Files in this format can be downloaded to your iPad or iPhone using iTunes. Unlike other downloaded eBooks that show up in DOCS, this one will show up in Books. Cloud services seem not to recognize this format or offer Kindle as the target.
If you unzip a AZK file you will find both source and binary data, generally duplicated in some fashion. This accounts for the increased size of the file over its ePub equivalent. There are some files with extensions and some without but the extensions do not mean anything except a possible reference to the source.
There will be three folders and 5 files. Much of this is conjecture after looking inside a sample and opening the files in a text editor.
- a manifest in JSON format
- a metadata file in JSON format
- a mimetype file in JSON containing references to all of the files in the gzip.
- a output file in JSON - basically nothing there, probably identifies errors.
- a rawmanifest is a binary file, likely MOBI format.
- a folder called resources. It contains JSON conversions of CSS, the original CSS, base64 images and the raw image (without any extension but unmodified from the ePub.)
- a folder called mobilocations - binary files with pointer to MOBI structures in KF8 binaries.
- a folder called frags (fragments) are the pieces of the book itself in a gzipped form. Inside these files is HTML stuff and partially compiled HTML likely in a mobi-container format.
It would seem that much of this could be left out and the eBook would still be readable.
It is likely that the delivered book by Amazon will have been trimmed of the excess. It has also been found that the zip file is delivered but the Reader unpacks the original file and leaves a set of files and folders in a eBook folder for the Reader to use.
 For more information
- MobileRead forum - contains a more detailed decode of this format plus a good discussion.