The book is published in four formats. Three are typical digital book formats: PDF, EPUB, and MOBI/Kindle.
The fourth is an APK file, which is an Android application. Specifically, it is the author's own digital book reader, originally written due to the low quality of Android EPUB readers.
As with the other formats, the APK file is distributed via the Warescription site. To install it, you will need to allow your Android device to install apps from outside the Play Store or other device-centric official channel. This can be set up in the Settings app. Usually, you can find an "Unknown sources" option in the Security area of the Settings app. Toggle this on, to allow installation from "unknown sources".
At that point, you can:
Visit the Warescription site on your desktop or notebook, then scan the QR code for the APK edition using Barcode Scanner or your favorite scanning app
Visit the Warescription site on the Android device itself, then tap the Download link for the APK edition
In either case, the APK edition should be downloaded to your device, at which point you can install it (e.g., by tapping on the download notification). Note that you may be prompted to adjust your settings to allow app installation from "unknown sources".
Another option, if you have the Android development tools set up, is
to download the APK to your development machine, then use
adb install path/to/the.apk, where path/to/the.apk is the path
to your APK file.
The app does not automatically update itself. This keeps your reading
environment stable until such point as you are ready to download a newer
version. So, just as you need to download fresh editions of the PDF,
EPUB, or MOBI/Kindle files, you can download a fresh APK. If you do
so via the barcode or by downloading the app directly on the device,
Android should indicate that you are upgrading the existing app. If you
elected to use adb to install the APK, you need to add the -r
switch to indicate that you wish to replace an installed app
(e.g., adb install -r path/to/the.apk).