If your code no work, there is some sort of an error
CSS
h2 { margin_top: 1em:text-align: center; page-break-before: always;}
I really recommend you
not do the 1 big book file thing.
It will be bog slow on low power devices
If you make an error
during copy edits , the whole book needs proofing
again, not just the actual chapter with the
original problem