Yuri!!! On Ice: Welcome to the Madness Side Story by Kubo Mitsuro
My rating: 4 of 5 stars
This came out with one of the DVD releases and is beautiful. It goes behind the scenes to how Welcome to the Madness came to be as well as Yuri's reaction to the Victuuri EX skate and how Otabek got dragged in to the show. It's not original in the sense that it doesn't expand beyond the current official story line at all, but it provides enough new content to the EX and events between the end of the Final and the EX that it's a wonderful piece all the fans seem to be enjoying.