Published in 1991 by Double Day Books for Young Readers.
I love the illustrations in this book. I couldn't find any information about Darke except that she illustrated several books in the 80's and early 90's. Thumbelina seemed to be popular at this time... the animated movie by Warner Bros. Family was released in 1994 and there were a couple other low-budget animated versions made in the years earlier. I always thought it was a rather melancholy type of story, but the idea of small people is always interesting to kids.