I started reading this book for the first time, in the clueless way of young boys, without realising that it was part of the Narnia series. My surprise and delight once I realised was priceless and remains part of the charm of the book for me.
I'm not sure in what order I read it, and despite my advice about reading in strict chronological order, this could be read at anytime after reading the first book. There are element in this story that connect the first book in the series with the last, but the story stands on its own feet. Lewis could have written a dozen such tangential stories and they would all have been fabulous. However he only wrote the one 'side story' and you will have to enjoy it for what it is, terrific.