Die Katakomben von Paris (Die 39 Zeichen, Band 1)

The ending hints at more adventures to come, but I wonder what motivated Rick Riordan to start the series this way and how he plans the story arcs.

The riddles and mysteries made the story so immersive, but I'm curious if they were inspired by real-world puzzles or entirely made up.

The book has funny moments that break the tension, but I wonder how Riordan decides when to lighten the mood versus building suspense.