Racso and the Rats of NIMH

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

Since it's very easy to misread, it should be explicitly mentioned that the book is actually titled Racso and the Rats of NIMH (note order of 'cs' in the first word).

During the course of the story, we find out that the strangely spelt Racso is in fact Oscar backwards.

IIRC, he adopted this variation because he didn't much like his real name. Admittedly it has been quite some time since I've read the story, so I may be wrong there.