One reason children like "pop novels" is that they often run in a series with familiar characters.

When Robert O'Brian's Newberry-winning book "Mrs. Frisby and the Rats of Nimh" was followed by a sequel, "Rasco and the Rats of Nimh," young readers clamored for more. O'Brian's daughter, Jane Leslie Conly, who wrote the second book, now has the third in the series. Young readers will be delighted."R-T, Margaret, and the Rats of Nimh" has the same redeeming qualities of the previous two books about super-intelligent rats who escaped from a famous research laboratory at NIMH and founded their own colony. In this one, two human children are lost in Thorn Valley where the rats have made their community. R-T (short for Artie), a runty asthmatic child who does not speak, is helped by the fat older sister, Margaret, who is arrogant and mouthy.

It's more than just the story of the children being saved by the young rat, Christopher. It's about the transformation of the children from misfits to people who demonstrate feelings and work hard as they learn about others. Even R-T talks a little as they interact with the rat community.

"R-T, Margaret, and the Rats of Nimh" is also about secrets; R-T and Christopher's secret club (with only two members) and Margaret's promise to never tell about Thorn Valley because the scientist, Dr. Schultz, is still seeking the whereabouts of the rodents: "We're the missing piece in a puzzle he's spent years trying to solve. Without us, his life's work will remain unfinished."

To me, this third of a trilogy is about communication and its various forms. The animals, themselves, can speak, of course; and the interrelationship with all the creatures in Thorn Valley is apparent. But most sensitive is the communication of what is believably a mute child who uses spoken language only when it is necessary but keeps a running chronicle on the wall closet behind the clothes: "Behind him the wall was covered with marks: loops and squiggles, straight and crooked lines. There was a mark for everything that had ever happened to Arthur, the good things and the bad ones, too. `I did it all by myself.' "

To those of us who realize the importance of young children's facility with environmental print (the printed word in their immediate world), R-T's squiggles and curlicues are a brilliant stroke in the novel. All the children will remember the times they wrote in such a manner, trying to communicate with others.

I suspect, too, that young readers will remember this cast of characters: the rats who wisely move their community out of Thorn Valley because they realize their secret can't be kept, and the human children and a caring newspaper reporter, Lindsey Scott, who believes the far-fetched story about intelligent rats.

Conly's latest work is more sophisticated and has a faster pace than the previous book. It is a welcome addition as some of the best fantasy of the century.

- Marilou Sorensen is an associate professor of education at the University of Utah specializing in children's literature.