Tolkien's version of the Kullervo tale has already been published (with commentary) in a volume of Tolkien Studies, but it's now being published in a new book, and so being made more easily available, or more well known perhaps, at least, to the general public.
As far as I have heard anyway, the commentary by Verlyn Flieger has been somewhat revised and Tolkien's essays on the Kalevala will also be included. Or at least one I'm guessing. I have the Tolkien Studies volume and there are two versions of the Kalevala essay printed there.
Verlyn Flieger talks about the new book on the BBC website, for anyone interested in that.