A book, as one document
EPUB is the twelfth conversion. A .epub goes in and the whole book comes out as one document: a section per chapter, in the order the book is read rather than the order its files are numbered, under the titles the contents gives them, with the pictures inside and the links between chapters reduced to their words.
Renaming an .epub to .zip almost works. It is a zip, and every chapter is inside it as XHTML you can open in a browser. What you do not get is the book.
The order is not in the file names
Open a real one and the chapters are called index_split_030.xhtml, index_split_002.xhtml, index_split_017.xhtml, in that order inside the archive. Those numbers are whatever the tool that built the book happened to write; sorting them shuffles a novel.
The reading order is stated once, in the package document's spine, and nothing else states it. That is where this reads it from — the same lesson a slide deck taught, where a slide dragged to the front keeps the file name it was made with.
The titles are the ones the book gives
A chapter's name comes from the book's own table of contents: the nav document in EPUB 3, the NCX in EPUB 2, both still in the wild. That label is what a reader sees in their own reading system, and it beats anything derived from the markup. Where a book gives none, the chapter's first heading is used, and its <title> after that.
What is dropped, and why
The cover. It is the jacket, not a page, and it would spend a quarter of the picture allowance saying what the title already says.
The address half of every internal link. A footnote marker, a cross-reference, a return arrow — each points at a file that is a section of this document now. The words survive; there is nowhere for the address to land. Notion and Confluence exports have been treated the same way since they arrived.
What comes through
The prose, the headings, the verse and the block quotes, the tables, and the pictures — carried inside the document as their own bytes rather than left in the archive. The author's name goes under the title, out of the book's own metadata.
A trilogy of about two and a half million characters converts in under half a second, in the browser. The book is not uploaded.