Digital BW, The Print

If you are using a RIP like QTR, then you might be able to use
Hammermill BusinessOne Glossy paper (I think that's the proper
name??). It doesn't like very much ink from these pigment printers, so
you will need to be able to change the ink limits and linearization.
It's fairly opaque for it's thickness, and it should work very well in
a "perfect bound" book. It's also fairly cheap at somewhere around
$9.00 for 300 sheets. I haven't done any long term testing because of
the ink limit issue.