Is there a sequel to Mrs. Frisby and the Rats of NIMH?

"Mrs. Frisby and the Rats of NIMH" was written by Robert C. O'Brien (a pen name of Robert Lesly Conly 1918-1973). This book was later reissued under a new title, "The Secret of NIMH" when the animated film of the same name was released. O'Brien's daughter, Jane Leslie Conly wrote two sequels, "Rasco and the Rats of NIMH" and "R-T, Margaret, and the Rats of NIMH."