I just spotted this question which reminded me of a short story with some similarities in English that I read in the late 1980s (it was reprinted in a primary school textbook as part of a comprehension exercise so I have no idea about the cover, I'm afraid). It definitely isn't the rats of NIMH.
It was written in the form of a series of letters written by a scientist documenting the rapid increase in intelligence of an individual rat or small group of rats on an island. I think he was experimenting on it/them in some way, but I can't be sure about that. The tone of his letters grows increasingly alarmed. The final letter, written in suspiciously broken English (possibly also described as in terrible handwriting), then says (paraphrasing) "all the rats have suddenly died out and I am about to kill myself, nothing to see here, probably best if you do not visit this island again".
Does this ring any bells with anyone?