I've seen the movie but never read the book. Meant to, but never did.
But I thought I remembered hearing something about a sequel, too, so taking a look around, this is what I found:
http://www.amazon.com/gp/product/006...lance&n=283155
It was written by O'Brien's daughter. It's not clear whether she conceived the plot herself, or was working from some notes he left behind. He did get a credit as co-author, so I suppose either could be the case.
"The Secret of NIMH" sort of runs together in my mind with "The Mouse and His Child." I do know that the stories are very, very different, but individual scenes and characters keep crossing over from one to the other amidst the clutter and chaos that I like to call my "mental processes."