If you've come this far into the series, I can't imagine skipping 'The Silver Chair'. It's kind of the dark horse of the set, and I mean that in the best way. The tone shifts noticeably from the earlier books; it's less about royal adventures in a familiar Narnia and more of a gloomy, almost desperate quest through some truly weird landscapes. Puddleglum alone makes it worthwhile—his relentless, hilarious pessimism is a fantastic counterpoint to the usual heroic optimism.
It deals heavily with memory, deception, and lost faith, themes that feel more mature. You don't get to lounge in Cair Paravel here. The journey is harsh, and the villain is psychological as much as physical, which I found compelling. For fans of the lore, it expands the world massively, introducing concepts and places only hinted at before. My only caveat is if someone loved 'The Voyage of the Dawn Treader' purely for its seafaring joy, this one might feel like a slog. But for depth and strangeness, it's essential.