The Last Kids on Earth and the Forbidden Forest by: Max Brallier

Picking up after Quint and Dirk’s Hero Quest, the gang is reunited—until a new threat emerges. The evil Thrull, alongside a sinister villain, is scheming within an other-dimensional fortress to build a mysterious Tower that could destroy their world. Jack, Quint, June, and Dirk must infiltrate the fortress to stop the plan before it's too late.