P. T. Ross

Pat Ross is identified by Penguin Random House as a children's book author whose works include Hannah's Fancy Notions and M & M and the Halloween Monster. Publisher and bookselling pages also connect the name with Meet M & M and other early-reader titles, suggesting a body of work aimed at younger children.

Reliable biographical detail appears to be limited in the sources I could confirm, so it is best to keep the picture simple. From the available publisher information, Pat Ross is best known as a writer of children's fiction, especially the M & M books.