From the flap:
"David Topman and Chuck Masterson, the young heroes of the three previous MUSHROOM PLANET books, have made an 'absolutely priceless' discovery, according to their friend Tyco Bass, the little astronomer-artist-inventor of 5 Thallo Street, Pacific Grove, California. It is a discovery that not only splits the world of science but proves to have fascinating and dangerous consequences for the boys and Prewytt Brumblydge. (Prewytt, you will recall, was the inventor of that marvelous machine called the Brumblitron.)
Mysteriously, Prewytt has been beset, lately, by a series of strange misfortunes.So now, with Mr. Bass far from earth and Prewytt Brumblydge delirious with fever, David and Chuck undertake a desperate and nearly disastrous journey through space to the Mushroom Planet on his behalf.
Fear of an ancient prophecy, terror and treasure in the City of Silence, the trial of Prewytt Brumblydge, and staggering revelations concering their friends on Basidium bring Chuck and David to a startling climax in this fourth suspenseful MUSHROOM PLANET book by Mrs. Cameron."
Spanish language edition:
Un Misterio Para Mr. Bass, Editorial Acme, Coleccion Robin Hood #193, 1974
The book was illustrated by Leonard Shortall, who is best known for illustrating Molly Cone's Mishmash books and Donald J. Sobol's Encyclopedia Brown series.