The Singers of Time (Used Hardcover) - Frederik Pohl and Jack Williamson
A race of turtle-like creatures conquers Earth, imposing a gentler set of values on humankind, outlawing destructive technology, and denying the validity of human scientific theories. When their home planet disappears into a black hole, however, the aliens' only hope for the future hinges on the possibility that humanity's flawed sciences might contain a glimmer of truth.