It's one of the 12 books about the "2920" year, the last year of the first era. Of course, this could mean nothing, but that is what they did with dawnguard. Possible DLC? Maybe.
Article on Bethesda's copyright: http://www.gameinformer.com/b/news/archive/2012/05/29/is-hearth-fire-another-skyrim-dlc-add-on.aspx


Afraid it's almost common knowledge by now!