calibre/e-books: added

wiki/calibre.md

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+# Calibre
+
+[Calibre](https://calibre-ebook.com/) is a cross-platform, free and open-source
+software suite for [e-book](/wiki/e-books.md) management.
+It supports various formats aswell as conversion between them.
+
+## Adding a Plugin
+
+A plugin by a third party can be added by selecting a `.zip` archive - usually
+named `<plugin name>_plugin.zip` - under Preferences, Plugins, Load plugin from
+file.

wiki/e-books.md

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+# E-books
+
+E-books are digital publications of books.
+They can be read with specific hardware - an e-reader - or on a personal
+computer.
+E-books are usually available in `.epub` or `.pdf` files.
+Amazon (`.azw`), Mobi (`.mobi`) and other brands selling e-readers have
+proprietary formats.
+
+## Management
+
+The management of e-books can be achieved by using special software suites like
+[calibre](/wiki/calibre.md).
+
+## DRM
+
+Proprietary formats may but don't have to feature DRM.
+There are some software solutions for handling DRM like the no longer supported
+[DeDRM by apprenticeharper](https://github.com/apprenticeharper/DeDRM_tools) or
+its supported counterpart [DeDRM by noDRM](https://github.com/noDRM/DeDRM_tools).
+Due to legal issues it is recommended to not use these or similar software.