I'm going to get straight to the point: the main revelation of the book is that Abeloth is one of the Ones. Those of you who watch TCW know that the Ones were three individuals representing parts of the Force: Daughter, the Light Side; Son, the Dark Side; and Father, the Keeper of Balance. According to the Killiks, the Ones were the masters and architects of the galaxy, what the Celestials became...sort of. It's all mixed up. (on a similar note, a Killik relief in the book confirms what was previously the widely accepted theory that the Corellian system was assembled using Centerpoint Station) Abeloth was apparently originally a mortal who was a servant to the Ones who took the role of Mother, and, envying their agelessness, she drank from the Font of Power and swam in the Pool of Knowledge. (which are both physical Dark Side nexuses and parts of the realm 'beyond shadows', which is more like a magical spiritual thing than any sort of sci-fi fantasy Force thing, in my opinion) This took place on Abeloth's Planet, in the middle of the Maw, where all the Ones originally lived. Discovering what Abeloth had done, the other Ones abandoned her, ordered the Killiks to construct the Maw to trap her, and moved to the Mortis monolith.
All this is both told in the book by the Killiks, but it's also revealed that Yoda apparently told Luke during the latter's training on Dagobah of Obi-Wan and Anakin's encounter with Mortis and the Ones during the Clone Wars, which seems to me like a very lazy way of linking the two series together than the author (writers?) didn't put too much thought into. The Jedi eventually report to Luke that the Killiks say that Abeloth is the Bringer of Chaos (the Destructor of Keshiri legend) who escapes every several thousand years to plunge the galaxy into an apocalypse and usher in a new era. Each time that happens, the Son and Daughter order the Killiks to reconstruct everything and build Abeloth a prison. Unfortunately, as TCW watchers know, the other three Ones were slain on Mortis during the Clone Wars, so the Jedi won't be able to get the Killiks to reconstruct stuff. At the end of the book, Luke sends ten Jedi out to search for the Mortis monolith and the Dagger of Mortis (a 'Force-imbued' dagger that can kill the Ones permanently, which, in my opinion, is just a really stupid magical artifact out of a fantasy video game that TCW writers decided to throw in. [or were told by Lucas to throw in, which wouldn't surprise me])
Luke tells the Jedi that the Father had asked Anakin to take his place as the Keeper of Balance, and the latter refused, and in that story arc the three original Ones were killed. Abeloth apparently wants to recreate the Ones, only with herself in the Father's place, taking Ben Skywalker and Vestara Khai and trying to make them drink from the Font of Power to become the Son of Light and Daughter of Darkness, respectively. (so now we have a magical font and a magical pool to give power and a magical dagger to kill those powerful beings

) Eventually, Abeloth's avatars are killed. One is killed by Tahiri Veila and Boba Fett. (Fett, by the way, is rescued by Tahiri from a webbed plant thing, which makes Fett look very not-so-badass in this book

) Another is killed by Saba Sebatyne when the Jedi, ah, sinks her fangs into the neck of a visual representation of a virtual Abeloth living inside the Jedi Temple's computer core. (it doesn't make sense to me, either

) Another is killed by Ben and Vestara in the Maw, when Vestara draws on the Dark Side power of the Font of Power (not drinking from it, just drawing on it) to overcome Abeloth.
The final avatar is killed 'beyond shadow' by Luke and an unidentified individual who wears spiked armor and has tattoos radiating from his eyes, which I suspected and Wookieepedia confirms is Darth Krayt/A'Sharad Hett. (personally, the whole story of Hett living for centuries and becoming the Sith Emperor and main antagonist of the Legacy comics series has always annoyed me and seemed like another attempt to mold a somewhat popular EU character into a badass Dark Side user, sort of like Quinlan Vos, who, for the record, I have never been a huge fan of, but that's all my personal preference) It is also revealed that Krayt is the dark figure that Jacen Solo saw on the Throne of Balance, but Jacen tells Luke (while 'beyond shadows', talking from the Magical-Fantasy-Pool-of-Taking-Unnecessary-Effort-to-Contact-the-Dead-Instead-of-Trying-to-Contact- a-Force-Ghost/Spirit-Thingy that he turned to the Dark Side and became Darth Caedus not because of the figure sitting on the throne, but because he saw Allana Solo standing next to the throne. Luke tells Jacen that by altering the future, he released Abeloth. (that makes no sense to me, either) Krayt tells Jacen that he didn't alter the future, but that he just delayed it. (Krayt, as we know, turns out to be right, since he becomes the ruler of the galaxy in Legacy)
I'm beginning to kind of see where the whole Ones thing is going: (and this is all pure speculation on my part, for now) Anakin, as the Chosen One, was taken to Mortis and offered the position of Keeper of Balance among the Ones. He was given the opportunity to bring true balance to the Force,
and he refused. He remains the Chosen One only in the sense of the Jedi prophecy. The Jedi legend of the Chosen One states that the Force is a light-side power, but that the Dark Side corrupts it, and the Chosen One brings balance by eliminating the Sith. In that sense, Anakin fulfills the prophecy when he kills Palpatine during the Battle of Endor, but that's only a very temporary restoration of balance, and he doesn't ever bring true balance to the Force. I really have no idea whether any of this is connected to the Throne of Balance. Seriously, it's all mixed up and very confusing. The current version of the vision of the Throne of Balance, by the way, as seen by Luke during Fate of the Jedi, shows Allana Solo sitting on the Throne as a Jedi Queen.
Anyways, moving past all the Force-magic stuff, the book has a couple other major occurrences: there is an attempted assassination of Allana by Vestara, which involves a thermal detonator blowing off the cockpit of the
Millennium Falcon. At the end of the book, the cockpit is replaced and Denning makes extra effort to mention the new adjustable seats in the cockpit, which, I think, is a very subtle way of showing that the galaxy has moved on from the death of Chewbacca. The Wookiee-sized copilot's seat that was a constant reminder of Chewbacca is now form-fitting and appropriate for humans. I kind of liked the moving on, even if Chewie, like Mara, was a character that should never have been killed off by anything other than old age, in my opinion, but what I didn't like was blowing up the cockpit in the first place. It's the
Millennium Falcon. You don't tamper with it. (other than the sensor dish, of course, which you are free to knock off or blow up whenever you want)
Also, by the end of the book, Vestara has accepted the fact that she is a Sith and Ben is a Jedi, and she leaves in Ship to find other Sith. Me, I say, "good riddance". Their teenage romance annoyed me. I can't put my finger on the reason, but it always seemed to me that it wasn't portrayed realistically. I'm also glad that she's gone, at least for now, because I was afraid that the writers would try to change her into the next Mara Jade, who, like all those things TCW has tried to create new versions of, simply cannot be replaced.
Additionally, the Jedi end up at odds with the government again, and end up leaving Coruscant again. I'm rather tired of all the political stuff surrounding the Jedi - another reason I think the OT was better: the Senate only exists for half of one of its movies. We do know the Jedi are based on Ossus by the time of Legacy, though, so it's a good time to start the transition, though I think that this is going to the make the Jedi into more of a secluded group focused on things that are going to delve even deeper in the almost-magical elements which I despise in Star Wars.
And, finally, the book ends with the marriage of Jaina to Jagged Fel, who leaves his job as Imperial Head of State. That's probably the one thing in the entire book which I had no real problems with. (I can't make a real judgment on how their romance was portrayed, since I never finished Legacy of the Force)
As a whole, the book was one big revelation that basically was there to tie up all the loose ends of the mysteries presented by the previous books of the series. Personally, I feel that a lot of the motives weren't really explained that well, and there was way too much of the almost-magical stuff included as an answer to all the mystery. It's like they decided to make all these characters go off to discover the greatest mysteries of the universe, and then just decided to say at the end, "magic did it!", which is an excuse, not an explanation. Furthermore, do we really need to know about all these magical Force-beings and Force-places and ancient species who constructed the universe? When all I knew was the OT, Jedi were special. The Force was special. The Force was not magic. The Force seemed unique to Star Wars. Now, not only are there thousands of not very special Jedi and planets full of not very special Sith, but neither is even that powerful compared to all these greater species that keep being introduced. Some mysteries are better left unexplained. When I first read the Corellian Trilogy, Centerpoint Station represented technology of ages past. Now, it's nothing more than a simple tool of magical beings whose powers are such that they near the status of gods. Star Wars, yes, is sci-fi
fantasy, not pure sci-fi, but that doesn't mean that all the technology should be dwarfed by magical powers. I don't need to know all the mysteries of the Star Wars universe and the Force. Characters can be special without the Force and without having magical, god-like powers. Han Solo is special. Chewbacca was special. Boba Fett was special. The droid duo is special. Leia was special before anyone knew she was Force-sensitive. And this is not just an OT thing, because I think that the entire storyline of the six movies made a reasonable amount of sense from a scientific point of view such that it remained clearly a part of the sci-fi genre. Some elements of Star Wars to me nowadays seem like pure magical fantasy, just with spaceships.
We don't need magical powers in Star Wars. To me, this book seems like writers decided to take all the things TCW did wrong and try to apply them to an adult, post-OT novel. Unfortunately, it appears that Star Wars is heading further and further into the magical realm of things. The end of the book clearly hints that Mortis is coming back, very possibly as the primary plot point of the next major post-OT novel series. I didn't finish New Jedi Order because the Yuuzhan Vong didn't really interest me. I didn't finish Legacy of the Force because I had no desire to read about Jacen's fall and Mara's death. But I know one thing, and that is that if Mortis returns as the central plot point of the next novel series, it will be the first post-OT novel series that I will have no desire whatsoever to ever begin reading.
Oh, and this book has Squibs.