Some additional notes on the volumes of The Chronicles of Narnia after the Pevensie trilogy, i.e., The Silver Chair (SC), The Horse and His Boy (HHB), The Magician’s Nephew (MN), and The Last Battle (LB):
1. While the first volume is self-sufficient (it is the Gospel in a nutshell) and a trilogy is found often enough, the next ‘good’ number (from a Christian tradition?) would be seven — and thus the subsequent four volumes, which was followed by numerous publications that speculated the inner logic of the series. Some have proposed that the series corresponded to seven cardinal sins or virtues or that each volume corresponded with a specific episode in the Bible, but most of the time these proposals broke down immediately (for example, regarding the latter proposal, it is easy to find correspondence for MN, LWW, and LB, but what are the rest?). Personally I am still convinced with Michael Ward’s proposal that the series embodied the Ptolemaic planetarian system where each volume was shaped with a specific planetary god — and for that you need to read his Planet Narnia.
2. SC still continued the timeline given in LWW, PC, and VDT. The setting is autumn 1942, a mere few months after Eustace and his cousins went into Narnia for VDT. Now he went to Narnia with his classmate, Jill Pole. The VDT film introduced Jill at the very end of the movie, when Eustace’s mom called him to go down and meet Jill who was waiting downstairs. We never met Jill, though, as the screen then turned into black and hence it gave you an ambiguous feeling. It introduced Jill, and it introduced her not. So the ending could function as a hinted introduction to the next volume, yet it is perfectly fine as well if the franchise was stopped here, since we have never really known Jill anyway.
3. HHB is different from the previous volumes as it does not follow the chronological order of the previous volumes. It was somewhere between the first and the second coming of the Pevensies, and hence was put between LWW and PC in the Harper Collins edition of the series. The uniqueness of the volume is also indicated by its independence of the story. Although SC does not feature any of the Pevensies, the main protagonist, Eustace, has been featured in VDT. On the other hand, in HHB these children are not featured at all and signifies a kind of rupture with the rest of the volumes (not totally, though, as the Kings and Queens are still mentioned by the characters in this volume). It perhaps indicated the independence of Narnia, which does not need these Kings and Queens to exist.
4. MN and LB, finally, formed the Alpha and Omega of Narnia. MN told the genesis of Narnia, and the main protagonist in MN is the professor in LWW. On the other hand, LB told the final eschatological battle of Narnia and hence its ending. I’m going to sidetrack here. Note that although MN told the origin of Narnia, it was not the first volume written and published. In fact, it was the next-to-last to be published. I must point this out as I still found some who found it difficult to imagine that Genesis 1 might not be the first chapter to be written in the Bible. It does not mean that Genesis 1 can not be the first chapter to be written. By no means. I am just highlighting the perhaps not so obvious fact that we must distinguish the final form of the text with its process of composition. Furthermore, the way the text is arranged might be different with the original form of the text, as is indicated by the Harper Collins edition of the series, which, perhaps somewhat outrageously, put MN at the very first! Not that this chronological order of Harper Collins edition must be inappropriate, though. It will have its own merits and demerits, I guess, if compared to the original publication order by Lewis. Returning to the Bible and Genesis 1, it means that there are many ways to interpret this chapter. You can try to locate it in its original setting (Babylonian exile?), but definitely you can try to interpret it according to its canonical position as well. Batman Returns before Batman Begins; how can it be?