Hai to Gensou no Grimgar volume 10 read:

This was an easy read because it was so good.  The truth about how our characters ended up in this world is revealed, and it's a doozy.  One of the girls is put in mortal peril for once, which I guess is to be expected seeing as how practically all the boys are already gone.  All in all a lot of interesting things happen this volume.  It's a must read among must reads.