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Calibre is an e-book library management application that allows you to read, edit, and convert your e-books directly from your Mac. It's a free, open-source, and cross-platform software that works on Linux, macOS, and Windows.

Here are some of the key features of Calibre:

1. Library Management: Calibre lets you manage your e-book library by adding, editing, and deleting books.

2. Format Conversion: You can convert your e-books from one format to another using Calibre's built-in converter.

3. News Feed to eBook Conversion: Calibre can automatically download news articles and convert them into eBooks.

4. E-Book Reader Sync Features: Calibre allows you to sync your e-book library across multiple devices, including e-book readers.

In addition to these features, Calibre also has a portable version available for Windows users. This means you can install the software on a USB drive and run it whenever you need to use it without affecting your computer's registry or installation files.

If you're looking to downgrade from a newer version of Calibre (5.x) to an older one, you'll need to use version 4.23 as that's the only pre-5 version compatible with the changed database format in version 5.

For more information and help resources, I recommend checking out Calibre's official website or online forums, where you can find user manuals, FAQs, and other helpful materials.

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