Well, if nothing else, I want to assure the reader that this book IS indeed vastly superior to other books in this series, particularly Star Wars: Fate of the Jedi: Ascension which in my opinion was an unmitigated disaster.
First, I wish to note that there will be plot spoilers in this review. I generally try not to spoil the plot, but some of my review is going to deal with direct criticism of the plot.
First, the plot moves. Thank heavens, the last book, and things definitely happen. Denning does a laudable job of making things happen. Characters (namely Abeloth) are FINALLY explained. Your mileage may vary on this. Personally, I found the explanation of what Abeloth was to be profoundly cheesy. It links in to a Clone Wars animated TV series that I think is just terrible, but apparently it is very popular for some so that is just my taste, perhaps... Regardless, though, I think the detail it went into warped her character. By the end, I couldn't really shake the impression that rather than Space Cthulhu, Abeloth was just some weird depressed stalker who had family issues.
Characterization is much better in Apocalypse. The things characters do actually make sense. And in the case of some, it isn't all good. Characters who are traumatized react appropriately. I really appreciated this from Denning, it is something often not found in sci-fi/fantasy.
Allana. There are significant spoilers here
The other reviewers who mentioned her are right. Her characterization has to be some sort of joke by Denning. It blasts way beyond the bounds of any sane credulity or suspension of belief into complete farce. I can't for the life of me imagine how this ever got past an editor's desk.
1. Allana is 8, but she is debating force philosophy, galactic politics and ethics with her grandparents as if she were a jaded adult.
2. Leia literally thinks this about her: "Her nine-year-old granddaughter was already a veteran of several assassination attempts and practically an old hand at close-quarters combat". This is absurd. She is 9, and Denning gives us lovingly described scene after scene of this little girl going Rambo on Sith and butchering them.
3. She gets on her pet in the middle of battle, breaks cover, and makes her pet charge the enemy while she shoots at them. This tactic works.
I'm sorry, but this is easily the worst characterization I've read in any book ever. However you choose to write your 9-year old girl characters, an adult thinking and speaking, Sith-butchering combat expert is the least believable I can think of.
The ending also is confusing. It involves a "dark man" who isn't identified by name, but described in some detail as if I should recognize him. I don't understand. I wonder if this is some tie-in to that comic series I haven't read, but have heard is set in the Star Wars universe far future. This strikes me as lazy writing. If you are going to include a character that makes no sense unless you read some obscure comic series, it shouldn't be playing such a significant role. I'm not a writer, but even I know that a basic maxim is that books should internally coherent and tell a story in themselves. This book fails that test by constantly throwing out references that forced me to resort to Google to understand.
The resolution of Ben and Vestara is annoying. Vestara, is basically treated as an utter pariah, literally sent into battle unarmed, and the Jedi have the audacity to act outraged when she gets fed up with them? Honestly, I feel like it was billed as a "big betrayal", but not really. She chose not to do a stupidly noble suicide, and half-heartedly led a Sith attack in a battlezone on Allanna... Who Vestara thought would be on the other side of galaxy, safe, as the heir to a wealthy, powerful interstellar kingdom.
In summary, while the events of this book moved rapidly to a conclusion, wrapping up multiple plot points, so much of what happened is nothing more than the result of sheer stupidity from the protagonists. Vestara's betrayal is definitely the result of her being systemically treated like scum. Allana is either a complete freak of developmental psychology or a joke by Denning, either way, reading her makes me grit my teeth, the ending with Abeloth is unsatisfying, links the story to what I consider some of the cheesiest canon material in the entire EU.
Finally, and astonishingly enough, the publisher is trying to pull a fast one on buyers yet again: the last ~20% of the book is promotional material. That's right, a good fifth of the pages in your book are length advertisements and excerpts for upcoming Star Wars books. I consider this a highly negative point, deceptive on the part of the publisher, and would highly recommend not purchasing this book on the basis of that. To clarify, looking at the Kindle edition. The book ends at 78%. That means 22% of the book (nearly a QUARTER) is taken up by appendix fillers and reams of advertisements.
In the end, Apocalypse is only good to wrap up the complete disappointment and disaster that was the Fate of the Jedi series. It resolves a few plot-lines at long last, and unfortunately has nothing going for it outside of plot resolution. You can easily get what matters from a brief plot synopsis on a blog, and if you really want to read this book, I would recommend getting it from the library, so as to avoid buyer's regret.