The Horse and his Boy - which is the fifth book in the Narnia Chronicles by publication - and chronologically a 'plot loop' insert within The Lion, the Witch and the Wardrobe, happening while the children are grown-up Kings and Queens of Narnia - is probably the most sheerly-enjoyable, fluid, coherent and well-structured of all the series.
And this, despite that it could easily be omitted without significantly affecting the understanding of the other six volumes. THAHB is essentially just a great story, on several levels; wonderfully told.
And yet it was the last Narnia book I read; because I was put-off by the Middle Eastern setting. The illustrations by Pauline Baynes are (as usual) very good; but I did not much like the 'exotic' subject matter...
Therefore, my enjoyment has been greatest when hearing the book, rather than actually reading it.
My introduction was via the superb 1990s Brian Sibley dramatizations for BBC Radio. After listening to this a few times, I moved onto the Audiobook version - which was perfectly performed by Alex Jennings.
(In Michael Ward's Planet Narnia scheme - this book is ruled by Mercury; and this is reflected in various subliminal ways by plot, character and symbolism.)
All the Narnia volumes have some particular Christian moral aspects - and in this the striking one is that Aslan make some direct interventions into the lives of characters; which he then explains to them. I take it that Lewis is telling us that we can each understand the specific workings of divine providence in the details of our lives - assuming was are sufficiently aligned with Christ/ Aslan (presumably through prayer) and ask the right questions.
But equally importantly, when the characters ask about how divine providence has operated in the lives of other people; they are informed by Aslan that "No one is told any story but their own."
So, on the one hand - we can understand why bad things happen to us; but on the other hand, we cannot know why bad things happen to other people.
This is a lesson that modern people (including non-Christians) would do well to reflect upon; since moderns (influenced by the arts and media) are always trying to discover 'why' some large and general Bad Thing happened to other people.
To make things worse - these events are often remote in space and time, and known only secondhand by unreliable accounts. Such are the status of ill-formed questions such as 'But why did a supposedly-loving God allow'... some particular war, genocide or plague - or disasters generally? Then, when a brief and wholly-satisfactory answer is not immediately forthcoming; this exchange is taken to have refuted Christianity...
At the level of atmosphere, this book is congenial to me in that it begins in the parched deserts, and the characters yearn for the green and pleasant 'North' of Archenland and Narnia.
When the two main characters - who have never known anything but a Middle Eastern climate and vegetation - approach and experience the recognizably European- then British-type landscapes from the burning South; I experience a renewed appreciation for the effects of rainy places!
This is an excellent example of the capacity for 'refreshment' found in good Fantasy literature; which Tolkien describes in his essay On Fairy Stories.