Welcome in from the lurk, CH.
Surely you aren't using <em> to represent an em space, are you? That's the html tag for emphasis...
Try & #8195; (without the space between the & and #, of course) (the Unicode value) or & emsp; (without the space) (the named entity)
I don't mean to be snarky; I've had my share of silly errors while learning book conversion... Hmmm, that didn't help, did it?