"100 Best Android Apps of 2019" - PCMag
(https://www.pcmag.com/article/362295/the-100-best-android-apps)

Calibre Companion (CC), recommended by calibre developers, is three applications in one:
- a superb device-based calibre library organizer
- a high-performance WiFi-based calibre device emulator
- a calibre content server and cloud provider (Dropbox, Box, Google Drive, Microsoft OneDrive, and Amazon Cloud Drive) client

1) CC is a device-based calibre library organizer. Once you put books on your device using one of the connection methods described below you can:
- view a book's metadata (author, tags, series, rating, dates, etc.).
- sort and group books by their metadata.
- see book lists by cover or by list.
- mark books read and sync this information with calibre.
- launch your book reader application to read a book (works with most reader applications except for Amazon Kindle for Android on non-Fire devices).

2) CC can connect to calibre over WiFi and be detected by calibre as a device. Once connected, you use calibre to move books to and from your device. Calibre automatically updates all the book metadata stored by CC to match the latest in your library.

3) CC can connect to calibre’s content server or to your cloud provider to browse your library and download books to your device. Cloud providers currently supported: Dropbox, Box, Microsoft OneDrive, and calibre libraries on the device or on an SD card.

For more information see our FAQ (http://cc_faq.multipie.co.uk/)

Notes:
1) CC is not a reader app. You must install and use one of the many excellent reader apps available.
2) CC requires that you use calibre calibre 0.9.02 or later, available at http://calibre-ebook.com/. Some CC features such as syncing book read information require newer versions of calibre.

A free demo version of CC is available in the Google Play store. Install and test with that version if you have concerns that CC won't work on your network or isn't what you need. To get it, go to http://www.multipie.co.uk/calibre-companion/ and click the button for the demo version (links to Google Play don't work here).

Permissions explanation:
- Allow Wi-Fi multicast reception: multicast is required to be able to find calibre without requiring you to enter an IP address.
- Users with Jellybean (Android 4.1): You will also see a permission about “Test access to protected storage”. This is caused by a change Android is making regarding SD card access, and simply means our app can save books on your external storage.