Just had a lengthy exchange of e-mails with Steven. VERY nice chap indeed. He confirms what I suspected, which is that it's the usual US/UK distribution rights business. There's a separate UK publisher who doesn't "do" eBooks. He's going to "encourage" them to do so, he says
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What an extremely nice author. I'll certainly buy "Empire" as a pbook even if I can't get it as an ebook.