The initial question made me think of 'world-building bibles' and specialized software, but after trying several for a Narnia-inspired project, my conclusion is more practical. I don't think you need anything fancier than a solid outliner and a good visual reference board. Scrivener's corkboard and folder system is excellent for tracking parallel narratives across worlds, like the Pevensies' London life versus Cair Paravel. Obsidian works well if you prefer linking concepts thematically—connecting the Deep Magic to the Stone Table, for instance. But honestly? The tool that helped most was a simple physical map I drew, just to remember which direction the lamp-post was from the Witch's castle. Digital tools are great for organizing, but the 'Narnian' feel came from letting the logic of that world dictate the structure, not the other way around.
I saw a post once from someone trying to use Campfire Pro for all the mythology, which seems overkill unless you're writing an entire new set of histories. For most projects, a dedicated character template for each sibling and a timeline to track their aging across books is the core need. Most word processors can handle that. The real support came from reading Lewis's letters about his process—he wasn't using complex tools, just a strong mental image and theological framework. So I'd say spend less time on tool features and more on clearly defining the rules of how magic and time flow in your version. The tool just holds the notes; the thinking makes them Narnian.