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I can only find one digital artbook published by Houbunsha (黒田bbイラストコレクション), and it's one whose physical release was just this year, so that doesn't garner much confidence they'll update their digital catalogue with past artbooks unless the one I just mentioned is them dipping their toes into the potential of digital versions for their artbooks.

Out of curiosity, where are you getting your digital books?
I was using amazon.co.jp, but they've been making it harder and harder purchasing their kindle books while overseas. It used to be I could use my forwarding service address and a VPN to buy books, but now that hasn't worked. Not sure if they updated it to decline overseas credit cards like DMM used to do a decade and a half ago, or what.