What's the Name of That Book??? discussion

I read this book a while ago, maybe 6 years ago, but I do remember some things. It's a children's novel in which an aunt sends her nephew a series of postcards which hold a secret code deciphered with the page, line, and letter numbers of various parts of The Magician's nephew. It's fiction. It's not Piranesi and not the Narnia Code. Thank you all for your help!

The Nebula Secret by Trudi Trueit

Thats's it! I've been looking too! Thank you so much, Orsolya Könyvtáros!!!!!!!!!!!!!!!!!!!!!!!!!!!

Just so we don't get confused, Moira is the OP (original poster).

Yes she is, I just got excited.

Glad you finally found it, Henry! Moira, is it the same book you remember?

yes, thank you!

You're welcome!