[Turtle91]

The Calibre ePub-split plugin will definitely do it...but there is definitely a learning curve. If you have a book where each chapter (I'm assuming an html page) is its own book then it just might be easier to do the manual method:

- change the filename extension from .epub to .zip
- copy all of the html chapter files to another folder (hopefully they have intelligible names)
- change the filename extension back to .epub
- open the epub in sigil
- delete all of the chapters after the first one, leaving all the other front/back matter, css, font, image, and ncx/opf files the same.

- save as (your first book name)

- delete the first chapter
- 'add existing files' using the white plus on a green circle icon (or File/Add/Existing files... in the menu) and add the 2nd chapter html from the folder you copied them to
- repeat the save as with your second book name

Repeat the last 3 steps for each chapter file.

You will have to clean up and edit each book regardless of the technique you use... the epub splitter plugin doesn't do most of the follow-up editing at all.

OBTW - I would HIGHLY recommend splitting that single file book into separate files, so each chapter is its own file...makes things a lot smoother in general.