Simple return on equity issue-BV or Market value

maybe i am just brain jammed but when we talked about ROE, are we dividing NI by BV or MV?

Book value.

ok,so we are really talking about for the original equity investor the NI per amount they invested

This brings me to the question about book value of asset where we only count BV of Long term debt when calculating BV of asset, why is that and why is short term debt that different