There are 3 books in the Rats of NIMH series. The newest book is R-T, Margaret, and the Rats of NIMH which came out in 1990.

1
Mrs. Frisby and the Rats of NIMH

By Robert C. O'Brien , Zena Bernstein (illustrator) ,

Book cover of Mrs. Frisby and the Rats of NIMH
Some extraordinary rats come to the aid of a mouse family in this Newbery Medal Award–winning classic by notable children’s author Robert C. O’Brien.

Mrs. Frisby, a widowed mouse with four small children, is faced with a terrible problem. She must move her family to their summer quarters immediately, or…
When you buy books, we may earn a commission that helps keep our lights on (or join the rebellion as a member).

2
Racso and the Rats of NIMH

By Jane Leslie Conly, Leonard Lubin (illustrator),

Book cover of Racso and the Rats of NIMH
‘Racso, a brash and boastful little rodent, is making his way to Thorn Valley, determined to learn how to read and write and become a hero. His bragging and lies get him off to a bad start, but a crisis gives him the opportunity to prove his mettle. A worthy…

3
R-T, Margaret, and the Rats of NIMH

By Jane Leslie Conly, Leonard Lubin (illustrator),

R-T, Margaret, and the Rats of NIMH
Lost in the North Woods, Margaret and her shy, mute brother, R-T, are saved and nurtured by the rats of NIMH, but a return to civilization could mean the end of the world of NIMH should the children divulge the secret of their rescue