An EPUB file uses XHTML to construct the content of a book. Unlike PDF, an EPUB file is based on a collection of compressed files, which can be actually edited. We will manually deal with these EPUB files and eventually put them in one EPUB file.
Download & install Sigil.
1. Decide which is your major EPUB file. Or just select the first chapter (EPUB file).
3. Double click one extracted folder, then "OEBPS" -> “Text”, you will find files like "html"/"htm"/"xhtml". Delete redundant files about cover, author bio, forward and so on.
4. Copy content html files to a newly created folder (anyname). Then repeat this operation on every other "non-chapter1" folder. Here are 4 epub files in total, you need to rename the other 3 files to zip and get 3 folders exclude the main (chapter 1) file. Then you will get all non-primary htm/html/xhtml files in one folder.
5. Open the primary EPUB file (Chapter 1) with sigil.
6. Right- click "Text" folder on the left column. Then "Add Existing Files" and locate to the newly created folder and add all those files.
7. Then you can drag and drop each html files and make sure they are in chapter order.
8. Double-click each file under Text folder and make sure the title is in "h1" format.
Select "Tools" menu -> "Table of Contents" -> "Create HTML Table of Contents". Then a "TOC.HTML"will be created under "Text" folder. Drag it to the proper position.