Indeed. We desperately need the "INTERVIEW WITH 騎士団長" book to complete the whole interview series. The problem is that we can't buy it even if we want to. It's simply not on dlsite. And the physical copies are so rare, they simply can't be found in 2nd hand market either.
For those who's interested. The Die Grünewäld Riterrorden book provides at least some details of the Knight Captain's captivity. Not much, but it's better than nothing I suppose...