When I was young I got given the complete Chronicles of Narnia box set as a present. Because I wasn't quite such a dedicated reader in those days, it took me quite a few years to reach this, the final book in the series.
Reading it again, and understanding how the books are all Christian allegories, it is fairly easy to tell that this represents the book of Revelation.
The opening of this book is somewhat unconventional. Whereas nearly all the books in the series open with children in our world usually facing some sort of personal trial, before ending up in Narnia, this one starts off in Narnia and introduces two talking animals; Shift the ape and Puzzle the donkey. It is fairly easy to work out very quickly that Shift is one of the main villains of the story.
Shift and Puzzle discover a dead lion at the bottom of a pool and Shift promptly comes up with the idea that Puzzle should put on the lion's skin and pretend to be Aslan, the Jesus character of the Narnia series.
The book then introduces King Tirian, who commits a murder at the end of the second chapter in a fit of rage; it is one of the more brutal moments in the Narnia series, and feels unusually shocking for a kids' book. Tirian is promptly arrested and bought before Shift, who is now declaring himself "Aslan's mouthpiece", and bringing out Puzzle in the lion's skin, claiming him to be Aslan. He promptly orders King Tirian to be tied up.
It is not until about a quarter of the way into the story that any familiar characters show up, as Eustace and Jill, previously seen in The Silver Chair are transported into Narnia. We also see Peter, Edmund, Lucy and also Diggory and Polly from The Magician's Nephew, but at first they appear only briefly when Tirian is somehow transported to our world in what he believes to be a dream. They appear again towards the end, but more on that later. The character Susan, who featured in previous books, does not appear, and is said to be no longer interested in Narnia, evidently representing someone who has abandoned their faith. Eustace and Jill are said to have been on a train, where they were going to meet the others; it was all part of a plan to use Uncle Andrew's magic rings (from The Magician's Nephew again), when there was a sudden jolt that they thought at first was a train crash, only to find themselves in Narnia.
The one thing I did notice when re-reading was that the loyal characters were known as "The seven friends of Narnia". The number seven is of particular significance in the Bible - as one of the church leaders I've met has told me, it is seen as a "perfect number". In the book of Revelation, there are seven lampstands representing seven churches.
The whole story is leading towards, as you can probably guess from the title, one final battle that represents Armageddon, and ends up in a complex three-way conflict. Reading the book, I started to see if I could work out who everyone represented, and I guessed that Shift was either one of the false prophets mentioned in the New Testament, or more likely the "Beast out of the earth" mentioned in Revelation 13, whose number is 666.
There is another character who appears briefly in a couple of chapters - Tash, who takes the form of some sort of human/bird hybrid, and is probably the character in the series most likely to give kids nightmares. He possibly represents the Devil, or possibly is some sort of embodiment of God's wrath.
The story starts to feel like the darkest in the whole series, as the situation for the Narnians becomes hopeless, as the Calormenes start invading, and Aslan is nowhere in sight. The book's main threats are actually eliminated about three quarters of the way in, but this doesn't feel in any way abrupt. This makes way for the final segment of the tale, which you shouldn't read about if you've not read the book yet.
Peter, Edmund, Lucy, Diggory and Polly appear again, but they are dressed as Kings and Queens; Eustace and Jill are magically dressed the same time, and the book indicates that they appear different. Re-reading this, it occurred to me that this probably represents the "resurrection body" that the Bible refers to.
At this point, Aslan himself shows up, and starts leading them into his kingdom as Narnia starts to fade away and the stars fall from the sky. Anyone familiar with the Bible will recognise this as a representation of the second coming of Jesus and the new creation. The last four chapters all revolve around the journey into Aslan's kingdom, and on the last page comes the book's final twist.
Aslan reveals that the moment Eustace and Jill arrived in Narnia, they were dead. In fact the train really did crash and both of them, and Peter, Edmund, Lucy, Diggory and Polly were killed. When I first read the book, I was completely blown away by this. As I re-read this, I saw various hints at this; as well as the assumption that the train had crashed, Peter mentions that he thought the train was going too fast, and Jill wonders what would happen if they died in Narnia.
Overall, I loved this book and it made for a fitting finale to the Narnia series.