The Golden Mushroom Trilogy (Paperback) (ISBN-13: 9781619506206)

The Golden Mushroom:
Soon-to-be fifth graders, Paul Drummond and Billy Sparks' summer vacation at the beach with Lige Drummond, Paul's grandfather, is interrupted when Lige's best friend Jess Hubbard disappears, and the boys are off to find him in Shumbus, a strange land deep within the Earth.

The Revenge of the Critches:
Things did not stay peaceful long in Shumbus after Paul and Billy's summer adventure there. The Critches are out for revenge. The Golden Mushroom is in danger and so is the very city where the Shumians have lived forever. With Jess Hubbard planning to help the Critches, they are sure to succeed unless Paul, Billy, and Argo can come up with something fast

The Goshmen:
Paul and Billy make their third trip to Shumbus, the strange land within the Earth. When they arrive, Argo takes them on a journey to a land of people forgotten in time, who hold a key to the history of Shumbus... if only they can escape from them and get back home.