It becomes easy to see why this book has sometimes been placed as the first in the seven-book series. But personally, I like it where it is--retroactively answering questions the reader may or may not have thought to ask. (i.e. Why did the professor in TLTWATW not surprised by the childrens' claims? And why did he seem to know a thing or two about the potential passage of time within another world...?)
Such a different structuring and feel from any of the other books in the Chronicles of Narnia series. The tone is almost more speculative or sci-fantasy than solely fantasy. It strikes a chord that seems likely to have resonated with Madeline L'Engle not so many years later...
As with the majority of these books, it begins in England--and features a pair of children who are transported to another place. But unlike in the others, their translocation isn't by some happenstance, unconscious request, or act of fate. It is instead the result of a callous and cowardly "experiment" perpetrated by a would-be magician. (There are points where the flawed innocence and nobility of the children is pitted directly against the greed and manipulations of said adult--and with a chilling effect worthy of any contemporary works.)
In some sense, the fantasy/sci-fi worldbuilding here was well ahead of its time. Not only is it clear in this portion of Lewis' imaginings that there are many other worlds, a whole multiverse of them, outside of our own... but there is also depicted a place of many portals leading to these realms--a waystation full of waygates, if you will.
Lewis only meant to write <i>The Lion The Witch and the Wardrobe,</i> originally. That he ultimately wrote a 7-book series is remarkable. But what is most demonstrative of his boundless creativity is the fact that this book--and perhaps all the rest by extension--exist because a friend of his asked him how a lamp-post ended up in the midst of a Narnian woods. And the question intrigued Lewis enough that he spent five years on and off composing this particular literary answer.