
According to Wikipedia, this was Lewis' most challenging book to write, as he started it after The Lion, the Witch and the Wardrobe to explain the lambpost, but changed its structure many times.
Chronologically, this is placed as the first book of the series, but as I think it wouldn't feel as nice if I was introduced to Narnia through this book. It would spoil so many things in the next books! I don't think I would have the same excitement while I was reading The Lion, the Witch and the Wardrobe if I knew who the White Witch was and how she came to Narnia, or who Professor Kirke really was and about the wardrobe.