Oh man. That's a big ass question. I'm sure I sent an incomplete GOSPEL chronology to somebody here, and I even promised to send him the completed version once I had it done...never got to doing that. My bad.
I should check that chronology one last time for consistency and just put it on the Tripod site or something.
Anyway, what can I write here?
The Ruby Ring: it was supposed to cover the interval between the end of Takato's forty days of seclusion (mirroring a certain someone from the Bible...) and the beginning of the Holy War. September 2018, to be precise. It began with Ruki's awakening as Lucifer Diabolos, Takato's improved Omnivice and DC meeting Lillia-chan. A young Tamer, Isaac Tong, mysteriously wakes up in a damp alley in Tokyo, half a world away from his home.
The very beginning of the Holy War is a big sky battle between Takato, Takako, DC and Isaac Tong versus the Archdemon Moloch and his Digimon partner, Chimeramon.
The climax of the story occurs two days later, on September 13 (Takako's b-day!). Moloch unleashes a horde of Digimon modified in disgusting ways, led by a monstrosity: the very first of the failures from Project ADAM, turned into an abomination. It's a huge battle, and lots of stuff happens, most importantly the Digi-Destined's encounter with Taichi, Takato and Takako using Ninlil for the first time, and Suzie's bloody battle with Leraje, the Demon of Gangrene.
After the battle, Takako introduces herself to the Archangels as Takato's imouto, and hands over the Seeds of Sephiroth, which become cards. The Omnivice Traits (sans Ruki) meet for the first time, which leads to a few amusing situations.
And Takato still remains hidden.
I forgot to mention, the title, "The Ruby Ring," refers to Alice's wedding ring. It would come up in a school scene, and Alice would reveal that, yes, she's married. She does not mention to whom, though.
Digimon Battle Arena: It was something the Hypnos organizations had planned since the Daemon War and the Tamers' ascension to fame. Digimon still have detractors, of course, and there's the issue that we have children fighting for their lives. By showing what they can do in a controlled environment, and showing that they are in no way coerced to fight, the image of the Tamers as "protectors of the world" would soar. Digimon Tamers from all over the globe are brought to Tokyo, and some of the Digi-Destined also decide to join this interesting tournament. Finally, Suzie convinces Takato to join in the guise of MedievalGallantmon, the mysterious wild Digimon.
So, this book is pretty much fight after fight after fight. Of course, some very interesting things happen during the tournament.
Takako had sworn to kill DC should he not show any progress in his Supernal training. Well, she finally decided to follow on that promise. There's also the issue with the Russian Tamers, who also happen to be incarnated Divinities with an agenda of their own. The Archangels use their tournament as a chance to try out their new Instrumental Weapons. Suzie shows her answer to the issue of the arm she lost in the fight against Leraje, and she proves in the tournament that she's growing at a ridiculous pace (if she didn't fight who she fought in her first battle, she'd have probably won, or at least made it to the final). Takato and Takako use the fights as stimuli to help them understand their supernatural powers. And, of course, Moloch chooses to send out his Second Chimera (the second Project ADAM reject) just after the final match. After the fight, in which Rika put everything she had (literally), she's too exhausted to fight the monstrosity. Takato is the only one who can save her, at the cost of his secrecy. The inevitable meeting would finally take place.