Wind Rider (Future Earth Chronicles Book 9)

Azur left the Siberian Rainbow Town with the wanderers, as an interpreter and secondary hacker who joined the Rainbow Town Freedom Team out of curiosity, then the Rainbow Towns Band after the aliens' defeat. The band's World Tour is over and Azur wants to go home, as do Toby and Gaia. From Paris to Siberia by steam-train, they reach his hometown where they start building their own airship, to be able to move around more easily. People keep vanishing from Rainbow Towns, and Azur and the crew must investigate, be it with Airship Siberia or the Wind Rider.