3.2.3: Publication date, or cover date in the case of a dated periodical, takes precedence over copyright date.If the book (including an e-book) has a publication date (usually included with the fine print on the copyright and printing information page), that date takes precedence over anything else, including the copyright date. If there is no publication date, the copyright date applies.
The rule for books is exactly the same as it is for magazines. Even if the "January 2016" issue of a given magazine might be on sale in November 2015, works published in it are 2016 works for Award purposes. This is not a new rule.
Now I am not the person who makes eligibility decisions about works for this year's Hugo Awards. What I say here does not have the force of law. But in general, a work's official publication date, even if that date is in a different year than its copyright date or the date it was first available to be seen/read, is what matters for Hugo Award purposes. So there may well be works that people were able to read/purchase/view in late 2015 that are 2016 works and not eligible for this year's Hugo Awards. You have to read the fine print.