I have written a number of times about calibre (start at https://duckduckgo.com/?q=site%3Aeogn.com+Calibre&t=brave&ia=web to find the past articles). calibre (always spelled with a lower-case “c”) is a popular and FREE app for reading and even editing ebooks. It does for electronic books just what iTunes does for music, allowing you to manage your digital book collection while offering excellent support for converting books to different formats and editing their metadata.
With calibre you can take an e-book in one file format and convert it to another that is supported by your e-book reading device and, if you’re not happy with the result, you can tweak the conversion settings and even manually edit the book’s contents and formatting. For instance, you can convert a PDF file to ePub format or to any of a number of other file formats. The result can be read on a Kindle, an iPad, on Windows or Macintosh or on most any other computer that has a screen large enough for reading ebooks. The calibre software is available for Windows, macOS, and Linux.
As described on the calibre web site at https://calibre-ebook.com/about#history:
calibre is a vibrant open-source community with half a dozen developers and many, many testers and bug reporters. It is used in over 200 countries and has been translated into a dozen different languages by volunteers. calibre has become a comprehensive tool for the management of digital texts, allowing you to do whatever you could possibly imagine with your e-book library.
calibre 4.0 has now been released. It includes several features including new content server capabilities and a new e-book viewer which focuses on the text by hiding the tool panel in the right-click menu.
With calibre 4.0, the content server has gained several new features. The most notable change obviously is the much-improved e-book viewer. Also, users connected to calibre via the content server now have the ability to edit metadata, convert books to other formats, and add and remove books and formats. To use the content server, just tap Connect/Share and start the content server. From there, you can connect to the provided address from other machines on the network. This should be an excellent tool for use in both public and private libraries, offering access to an ebook to multiple simultaneous users.