Wow, fucking time travel AND zombies? Maybe this series is moving too far, too fast.
Anyway, the novel is pretty good overall, but it seems to stray too far from the original vision of the series, just not to the positive extent of Nekomonogatari (White). Banter is somewhat downplayed in favor of actual plot progress, and this is realized by having Araragi and Shinobu travel back in time as something of a joke, with Shinobu suggesting the act in order to help out Araragi, who has procrastinated doing his summer homework all break. There is thus a little too much dialogue focused on arguing the nature of time travel, arguing on its logic, when ultimately Araragi seems to stop actively caring about time paradoxes and simply goes with the flow, leaning too heavily on the metafictional elements of the series to eschew any serious consideration of the implications of time travel. As is reasonable for the series, many bad things seem to be directly causally related to Araragi and Shinobu's actions, only to be revealed to be safely otherwise just pages later. And in the end, we take the Dragon Ball approach (Trunks is explicitly referenced in the novel) of having time travel occur in parallel universes, with the initial time travel in this novel being to a completely alternate universe set in a "past" reminiscent of the "past" of Araragi's "real" universe, whereupon the heroes, attempting to travel back to their "present" accidentally travel to the parallel universe's "future" (parallel to Araragi's original "present"), much as how Trunks travels back to fight Androids 17 and 18, only to encounter 19, 20, 16, and Cell, the latter from another "future," with Trunks eventually returning to his own timeline in order to vanquish an Imperfect Cell. Somewhat similarly, Araragi and Shinobu plan for a big showdown with a powerful enemy in order to save the parallel universe, having no apparent consequence for their timeline, only out of duty to Oshino and Hachikuji.
So the episode's title, "Mayoi Jiangshi," seems to suggest an overt focus on Hachikuji, much as the previous volume focused on Hanekawa to enough of an extent that she was even the narrator of that novel, but what we really get is another "Tsukihi Phoenix." That is, Hachikuji is mostly a plot device in this novel, not actually actively featured very much at all, just as "Tsukihi Phoenix" spent more time on the other sister, with Tsukihi herself getting merc'd halfway through, and the rest of the novel focusing on a fight against series newcomers Kagenui and Ononoki. Here, Araragi begins by searching for Hachikuji, having hung out with her at his home, where she accidentally left her backpack. He encounters a benign Ononoki, who muses on aberrations' interest in existence, including Araragi's undeath, her own rebirth from having previously been a corpse, and Hachikuji's lingering presence as a ghost. Hachikuji herself remains absent from Araragi's line of sight, though she stays on his mind when he time-travels with Shinobu, his thoughts of Hachikuji, we later learn, being the directions to which the time travel steers, and Araragi and Shinobu end up in a world eleven years in the past, one day before the Mother's Day during which Hachikuji was supposed to get killed by a random motorist. So the two, after some extensive dicking around, manage to save Hachikuji, and attempt to travel back to their present time. They believe fate may have ways of keeping certain events in place, to prevent too much time-fuckery, so it is assumed Hachikuji would have to be killed at some later date, possibly just hours after having been saved, but Araragi acted against this supposed futility, believing Hachikuji had a right to see her mother on Mother's Day, even if she'll still have to die.
Of course, things aren't good at all when Araragi and Shinobu return to the present, and instead they seem to enter a world devoid entirely of life. Looking for clues, they seem only to learn that everyone disappeared on June 14th, roughly two months in the past from their current present. Unfortunately still, it seems they cannot time-travel again because the shrine from which Shinobu gathered energy (the same shrine from "Nadeko Snake") has been sealed in this reality by a talisman that absorbs all aberrant energy, rather than disperse it, like the talisman Araragi and Kanbaru used in the "real" timeline. Unfortunately even still, it turns out no one's actually missing from the world, but that everyone has become an aberrant zombie that seems to exist only at night, in what seems like a pretty big shark-jump. After some consideration, Shinobu realizes the significance of June 14th and the saved life of Hachikuji: not having become a ghost, Hachikuji could not have met Araragi, thus Hachikuji was no longer there to notice Shinobu having ran away from Oshino's cram school building in June, thus Araragi would not have been able to search for the "missing" Shinobu, thus Araragi would have been killed by Black Hanekawa. It turns out the reason for Shinobu's running away from home was simply that she wanted Araragi to find her, the consequence being that if he failed she would kill all of humanity. With Araragi's death, the Shinobu of this timeline had no reason not to kill everyone (by her own logic), and, not being bound to Araragi any longer, she'd regained her strength, becoming Kissshot Acerolaorion Heartunderblade once more. With this increase in power, she sought to turn all of humankind into her thralls, enslaving everyone into joining her cause, the widespread death of humanity.
Our Shinobu believes the zombies to be the result of the other Kissshot having died, with her vampire servants thus degrading into mere zombies. Araragi and Shinobu later run into an older Hachikuji, in her early twenties, who hands them a later by the Meme Oshino of this timeline, spelling everything out for the reader in a massive info-dump, not nearly as magnificent as the similar letter in the previous volume, to date the series's high point. In the letter, we find that the past adventures of Araragi and Shinobu have reflected themselves as an urban legend, that this Oshino was easily able to learn that another Araragi-Shinobu set from another timeline time-traveled, that our Shinobu's prediction of events was indeed mostly correct, but that this Kissshot still lives, that the zombies were a result of her being too unfamiliar with creating thralls, a somewhat entropic thinning of her powers, that Oshino planned on teaming up with Kagenui and/or Kaiki to try to stop Kissshot (predicting that he'd died by the time Hachikuji handed over the letter), and ending his letter with a plea to save Hachikuji once more, saying it's useless to save a girl only to have her die so much later, saying it isn't necessary to stop Kissshot to save the world, but that she should be stopped to save this older Hachikuji.
As one may expect, based not only on previous installments but also on the page count at this point, we don't actually get a big fight against Kissshot. Of course, that's rarely the point of Monogatari. Instead, she gives up almost instantly. She'd clearly survived trying to kill herself by self-immolation via sunlight, having regretted what she'd done to humanity. The point of her rage was that she wasn't able to reach a true, positive connection with Araragi. Seeing the possibility as realized by the "real" timeline's characters, this Kissshot is perfectly find letting our Shinobu absorb her energy, killing Kissshot, freeing humanity from her control, and power Shinobu enough to cross back into the "real" universe, where everything turns out to be fine, except that Hachikuji's "meta" comments show far too much awareness of the events concerning her parallel self (as we should expect, given Hachikuji's character).
Anyway, it is clear Araragi has yet to catch up with the events seen in Nekomonogatari (White), so we have maybe a few novels before we catch up to the burning of the cram school, the reintroduction of Episode, and the introduction of Kanbaru's aunt.
There are neat, subtle references to the previous book, with a particularly interesting example being Araragi's respect for Mr. Hachikuji, whom he had previously believed was a less than savory father, thus learning what a father really is (not that Mr. Araragi is a bad guy, he just has zero screentime), in parallel to Hanekawa learning what a real father is in hanging out with Senjougahara.
Lastly, this installment more or less cements Shinobu as Best Girl, having come full circle from her Best Girl status as of Kizumonogatari (obviously before getting the "Shinobu" name), though of course Kizu- gave very strong love for Hanekawa as well, who was also fantastic in Neko- (White). And adult Hachikuji seems pretty cute, though of course we'd probably never see her again. So probably we can just rest easy with Shinobu, particularly in her guise as a little blonde Hanekawa.