Took me too long to read this, though I'm not sure why. Or, I do know why, which was simply that I got lazy and read manga or played video games instead, but maybe I was also slightly burned out on the franchise, despite having taken forever between Koi- and Tsuki-.
Anyway, this volume sees the beginning of the so-called "Final Season" (though there would be "Off" and "Monster" seasons to follow). Like with the Second Season novels, there's a greater focus on continuity than the series's beginnings. The case this time is that Araragi's reflection seems to have vanished, a spontaneous display of his vampirism spiking back up, determined by Ononoki, Kagenui, and Gaen to be the result of Araragi becoming far too used to feeding Shinobu in order to power himself up and brute-force his way through his past couple problems, in particular the month he spent going after the then-divine Sengoku. Ononoki had previously made herself into a firmer ally for Araragi, and this volume adds Kagenui as a more benevolent force than her previous appearance.
We also get a bit more into the experts' college years with Gaen-sempai, being now introduced to Tadatsuru Teori, a puppeteer and origami enthusiast who played an important role in the "creation" of the tsukumogami Yotsugi Ononoki, and whose slight conflict with Kagenui caused a "curse" for neither person to be able to walk on the ground. As with previous novels, Tsukimonogatari operates along the "formula" of backloading the book with the more "significant" elements of plot, so Teori is only mention shortly before he is formally introduced, and his introduction is within the last 20% of the novel, to be resolved as always before the epilogue ("or punchline") of the story, severely limiting his presence and thus the "weight" of the character's existence, or "the existence of the character himself," as the big thing seems to be to set up Ononoki to kill Teori to kinda-sorta-inadvertently prove her being a "monster" to Araragi so as to cause a bit of a schism between Araragi and Ononoki, as directed from the shadows by an unseen interloper, assumed to be the "Darkness" from past volumes, and teased by Ononoki earlier in this text to be Ogi.
Basically, this novel does a good job of linking the contents of the Second Season to what will be resolved throughout the ensuing Final Season. In true Monogatari fashion, NISIOISIN jerks us around a fair bit with banter and play before getting too deep into the "story" proper, but in a bit of a twist much of the dialogue in this novel is actually focused on the "actual" issues at the heart of the story (ie, Araragi's growing vampirism) instead of just messing around. It's hard to tell if this is better or worse than the "common" Monogatari practice of spending 60% or so of a novel's page-count just dicking around before rushing through the resolution in the end, as any reader of Monogatari should very much enjoy the series's idiosyncratic banter (in fact, I think an earlier page in this novel has someone mention metafictionally how the purpose of the series is banter over anything else). The comparative lack of banter here is hard to complain about when we get some pretty okay world-building as well as a hint into the consequences of Araragi's actions (and the implication of just how absolutely different Araragi is from his pseudo-mentor in Oshino).