To clarify what wallcraft said:
If an eBook explicity specifies its justification with, for example:
<div align=justify> ..... </div>
then you're stuck with whatever justification is specified.
Mobi's recommendation is that you DON'T specifiy it. ie just use:
<div> ..... </div> (or <p> ... </p> if you prefer)
since that allows the user to set the justification