User blog comment:Xhatefate/DxD Updates~/@comment-3965848-20140902164357/@comment-1359322-20140903163433

DX1. is a compilation of short stories much like Volume 8, 13 and 15 but it's not part of the main story, just merely compilations of short stories. Not sure about the full details but i think it had something to do with Ishibumi not wanting another SS compilation Volume on the current arc as it'll mess up the storyline and the fact that having another SS Volume will mess up the sales which did because of Volume 13 & 15, so it was decided that DX1. would be created instead to replace the SS compilation volume. So now there's no more SS compilation volume.

The book would be out on March 2015 as stated on your link, and an OVA included along (original story by Ishibumi, i think), so we can expect that Season 3 won't be earlier than March 2015 since the info on Season 3 will most likely be out with that OVA. ( ._.)