That's what I do now -- just delete the ISBN, try again, and accept the version with the most complete information. Of course, that gives me a listing that's not for the ebook version. I don't really care, but it seems odd to me that I wouldn't be able to find the correct information online. (I could also leave the ebook ISBN, and just copy and paste the other metadata from the Fictionwise web site. I'm just too lazy.
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It seems that other people would be having this problem, since Calibre is an ebook management tool, but maybe everyone is just doing what I'm doing. It occurred to me, though, that I should post about it just in case there's another way to get the accurate information.