Quote Originally Posted by K.S. Crooks View Post
If you haven't published book one yet then the prequel can be book one or book five. For the seven books of the Narnia series C.S. Lewis wrote The Lion, The Witch and the Wardrobe (book 2) first. He wrote The Magician's Nephew (book 1) sixth. Book one explains why the Wardrobe is magic, how Narnia formed and other details. You can read them in the proper order, but it feels nicer when read in the order written.
Worth noting that when I was a kid in the 1970s, The Magician's Nephew was published as Book 6 of the series.

Either way, OP: there's absolutely precedent for what you're doing, no matter how you number them.