Moderator Notice
I moved this, because I think Editing is the best solution.
ways (for EPUB)
a)You can use the editor (preview screen) to split chapters (into pieces)
b)CSS: page-break-before: always in the chapter title block style
AZW3 (output)
might honor these if you use Publishers styles on your device
There are Cheatsheets for both CSS2 and HTML out there.
They are handy, but not a substitute for knowledge
