Mrs Frisby and the Rats of NIMH by Robert C. O'Brien
They are not like other rats. They work at night, in secret . . . Time is running out for Mrs Frisby. She must move her family of mice before the farmer destroys their home. But her youngest son, Timothy, is too ill to survive the move. Help comes in the unexpected form of a group of mysterious, super-intelligent rats. But the rats are in danger too, and little by little Mrs Frisby discovers their extraordinary past . . .