Here's what Amazon has to say
Sylvester the donkey finds a magic pebble and unthinkingly wishes himself turned into a rock when he's frightened by a lion. How Sylvester is eventually reunited with his loving parents is a tender, magical tale of the transforming power of love. This Caldecott Medal-winning book is a modern classic beloved by children everywhere.
Author/Illustrator William Steig (1907- 2003) won numerous awards for many of his books. His classic style is indeed timeless. Fun Fact: Willian Steig wrote Shrek! in 1997, which of course became a major motion picture, plus a lucrative francise, a decade later.