Racso and the Rats of NIMH is written by the daughter of Robert C. O'Brien (who wrote the original Mrs. Frisby and the Rats of NIMH) Jane Leslie Conly. It concerns mostly Timothy Frisby and a rat named Racso, the son of Jenner (not Nicodemus), who had run away from home to rejoin the Rats of NIMH in Thorn Valley. Together they help save Thorn Valley from destruction by sabotaging a project to dam up a river flowing into the valley.

A decent book, all-in-all, but it doesn't quite match up to the original.

There is a sequel to this book, also by Jane Leslie Conly entitled R-T, Margaret, and the Rats of NIMH