Living up to its hype, or at least the hype of its anime film adaptation, The Disappearance of Haruhi Suzumiya is by far the best novel in the series since the first installment. Of the first four novels, only this and Melancholy accurately stand as proper narrative novels. Sigh tries to pose as a direct sequel to Melancholy after skipping six months' worth of events, and the novel's overall plot pales in its gravity compared to the battle of its predecessor, at once offering a more relaxed "slice-of-life" atmosphere to cool down from the high-stakes near-apocalyptic climax of Melancholy, while also seeming like a dud explosive, promising by relation to Melancholy to give the audience another wacky ride, while in actuality all the supernatural elements are incredibly tame and the conclusion is brushed through with a bit of a non-ending. Boredom is not even a novel, consisting instead of a handful of sequential episodes, subtly developing Nagato's character while not much else happens w/r/t Haruhi herself (and indeed the series could have benefited more, at this point, by changing its title to reflect the reliance on Nagato's plot-device powers). It is perhaps a little strange that Disappearance, though taking place chronologically after Sigh, mostly sidesteps the events of that second novel in favor of tying directly into the "Bamboo Leaf Rhapsody" story of the Boredom collection, the side-effects being a further diminishing of Sigh's overall worth w/r/t the series as a whole and a simultaneous augmentation of Boredom's own merit.
After nearly a year of the SOS Brigade's existence, Kyon finds himself apparently whisked away to another dimension, parallel to his original, featuring much the same cast, only in different roles, and with the unusual return of Ryoko Asakura, formerly an agent of Nagato's same Data Overmind, and who had previously made an attempt on Kyon's life back in the first novel. Curiously, we finally get something close to resembling a "slider" as potentially teased by Haruhi herself back in her original introduction (though, as it happens, Kyon didn't actually "slide" anywhere). In this different version of the world, the upperclassman Asahina has no contact with her kouhai Kyon, Nagato has changed from an emotionless alien robot to an incredibly shy, bookish girl who hides her insecurities with a mask of indifference falsely resembling her more familiar self, Koizumi's classroom straight up does not exist within this incarnation of the school building, and (in case you couldn't tell by the book's title) Haruhi seems to have disappeared entirely.
Thrown into this strange new scenario without explicit guidance from his paranormal friends, Kyon has no choice but to stumble through his life, hoping for a miracle to help him figure out what moves to make. When he visits the literary club room, former site of his world's SOS Brigade, Kyon (after accidentally almost-assaulting the meek new Nagato) is somewhat surprised to see an old computer sitting on a desk, an out-of-place artifact when no computer existed before Haruhi hijacked the room, and the one in SOS Brigade ownership was a high-end model regardless. He also finds Nagato's copy of Dan Simmons's Hyperion, used as a plot device in Melancholy, and it contains a bookmark with a cryptic message, just as it had previously - the message is different, of course, giving Kyon something of a clue to resolve his current situation. He is thus tasked with gathering "the keys" so that he might be freed of this new world, with a deadline of two days.
Tanigawa jerks the reader around for a couple dozen pages before finally allowing Kyon to learn of Haruhi's presence in this reality. Kyon's friend Taniguchi, having been in the nurse's office one day and absent from school the next, was (in)conveniently separated from Kyon while he screamed at Asakura for existing in Haruhi's stead. When Taniguchi finally shows back up, he reveals that Haruhi's been attending the school adjacent to Kyon's North High, leading the observant reader to question why Haruhi would have ever attended any other school when a mysterious stranger clued her in to the existence of aliens, time travelers, and espers in North High (that is, Kyon having time-traveled to help Haruhi create faux-Nazca lines when she was in middle school, three years prior to the story's "present"). Luckily for both protagonist and reader alike, the Haruhi of Kouyouen Academy responds immediately to Kyon's recollections of that Tanabata holiday three years ago, with his pseudonym "John Smith" being all the proof she needs (oh, by the way, Koizumu and the rest of his class from North attend Kouyouen now). Strangely, Haruhi recalls a second encounter with John Smith, later in the night after the line-drawing incident.
After gathering all the members of the SOS Brigade (including a traumatized Asahina), the old computer in the literary club room kicks to life to give Kyon a message from the "real" Nagato, presenting him with the choice to either stay in this mundane world or return to the paranormal chaos of his life with Haruhi and her Brigade. Obviously, Kyon chooses to go back to his own reality, having decided earlier in the novel that he actually misses Haruhi and her shenanigans. He is thus thrust back in time to the day of the Tanabata festival, encountering the adult Asahina just after she had previously spoken with his several-months-earlier incarnation, whereupon Kyon realizes he did not slide to an alternate dimension, but rather the world around him was reconfigured while only he retained the knowledge of its previous incarnation. The two of them later going to Nagato's house, after she'd frozen the several-months-earlier Kyon and Asahina (Small) in time, to be reawakened for the end of the "Bamboo Leaf Rhapsody" story from the previous book. We thus learn from Nagato that she cannot synchronize with her self from the December of Kyon's first year of high school (as she could with her July self), tasking Kyon and Asahina (Big) with fixing reality by striking the culprit with a special needle that would neutralize the power stolen from Haruhi and used to rewrite the fabric of reality.
The twist? Nagato herself was the culprit, having learned how to "feel" by hanging out with her friends in the SOS Brigade, having become overburdened from constantly righting Haruhi's own alterations to reality. Not only did she create a mundane reality as a backward gift for Kyon, incorrectly thinking he'd prefer a simpler life, but she also resurrected Asakura to act as a fail-safe, retaining her psychotic behavior but rewriting her allegiance to the Data Overmind as a deep dedication to protecting the new, milquetoast Yuki Nagato. Asakura manages to stab Kyon with her old knife before he has the chance to inject human!Nagato with alien!Nagato's cure. In a tastefully abrupt deus ex machina, Kyon is saved by a second set of Nagato-Asahina-Kyon, hinting a future event wherein Kyon must use Asahina to time-travel back to December 18 and have Nagato act once more as a human(oid) shield against a homicidal Asakura. Tanigawa thus reveals a larger-scale plan to have his characters time-travel back and forth, cementing the "Bamboo" story as integral to the series as a whole, signifying Boredom as one of the most important books for its inclusion of that tale.
It should go without saying that Haruhi is the most pleased when Kyon awakens from the coma that Nagato establishes to cover up the previous three days. In typical "tsundere" fashion, Haruhi pretends she merely cares for the well-being of her Brigade members, and unfortunately Tanigawa doesn't do too much to really make the reader feel her growing love for Kyon. But what's most bothersome is that human!Nagato is set up so perfectly as a cute little love interest, yet the same chapter where her shy cuteness reaches its zenith is the same wherein Kyon realizes he misses Haruhi's presence. The reader can only imagine the possibility that the malfunctioning alien!Nagato may have rebuilt herself as the demure human!Nagato to appeal to Kyon more directly, taking on a personality not unlike that of the childlike, easily-spooked Asahina. And perhaps Asakura existed to make sure Kyon didn't leave human!Nagato's version of the world. But alas, we are cuckholded out of a victory for Best Girl. And without any great flags raised for Haruhi herself, as would usually end the series's stories on high notes!
Postscript: Extremely fucking hyped to get around to the film adaptation, god damn.