That sounds like a solid framework. Start with a clear protagonist and a recurring symbol that carries the key. Every chapter could hide a cipher—maybe a simple substitution in the first few, then move to a transposition or a book‑cutter pattern. The plot must still move forward even if someone skips the decoding, so clues should also serve the narrative. I can sketch a skeleton with chapter beats and the type of code to use. Let me know if you want me to dive into specifics.