There is some light romance between Alex and Brandon, and it looks for a while like Brandon has truly perished, but the sensitive reader can be assured that he is saved.
There are lots of puzzles, codes, riddles, and runes for readers who enjoy working on these, and the idea that Jules Verne planned these all a very long time ago (in French!) is a fun premise. There is one scene that reminded my of National Treasure-- there are some structures in the cave that have been waiting for them for well over 100 years!
Hand this to readers who enjoy adventure tales with a bit of the supernatural to them, like Park's Forest of Wonders, Korman's Masterminds, Stokes 'Addison Cooke and the Treasure of the Incas, Gonzalez's Moving Target and Gemeinhart's Scar Island.