Every reading device opens one. Getting the words out of it is the part that is awkward.
Convert an .epub file to Markdown
An `.epub` is a zip archive of XHTML — a web page per chapter, a stylesheet, the pictures, and a package file that lists them and states the order they are read in. It is the open format the whole industry agreed on, which is why a Kindle’s `.azw3` and `.mobi` are the odd ones out rather than the standard.
Apple Books opens one on a Mac, an iPhone or an iPad, and Microsoft Edge opens one on Windows with no install at all. Calibre is the free desktop reader that also converts between formats, and Thorium is the one to reach for if you want a reading system that follows the spec closely.
A Kindle does not read `.epub` directly, but Amazon’s Send to Kindle accepts one and converts it on the way.
Because it is one. Unzip an `.epub` and every chapter is there as a file you can open in a browser. What you will not get is the order: the files are commonly named `index_split_030.xhtml`, `index_split_002.xhtml` and so on, and those numbers are whatever the tool that made the book happened to write. The reading order lives in the package file’s spine, and nothing else states it.
Converting it directly gives you one Markdown document: the chapters in the order the spine puts them, under the titles the book’s own contents gives them, the pictures carried inside the file, and the links between chapters reduced to their words, because a merged document has nowhere for them to land.