In 1959, Timmins illustrated the book Lassie: Forbidden Valley.  Forbidden Valley was written by Doris Schroeder, and published by Whitman Publishing.  Harry may have done the cover for this one.  This was followed by inside illustration work for Lassie: Treasure Hunter in 1960.  Treasure Hunter was written by Charles S. Strong and published by Whitman.  Harry did not do the cover for this second book.  Below are covers for both books and a representative illustration.

Although Lassie’s roots trace back to the 19th century, she was popularized by Eric Knight in a 1938 short story for The Saturday Evening Post.  This was followed by a full-length novel by Knight in 1940, and the 1943 MGM movie Lassie Come Home.  Subsequently, numerous Lassie books were written by various authors and published by various publishers.  Lassie was the subject of many additional movies, a radio show, and a hit TV show that ran from 1954 to 1973.  I watched (and loved) the TV show regularly as a child.