I always enjoy Marion Dane Bauer and I do think this should be included in the SSYRA. I liked the story because I do like mysteries with happy endings. I also liked the quick read. Recently I read the Outlander and Dragonfly in Amber by Diana Gabaldon which are both about a character that walks through stones back in time like Liz in this book goes back in time through the wall her her Grandmother's cabin.
I enjoyed the relationship between grandma and granddaughter....I hope many of the children make that connection to their real lives- if they have a Grandma. Liz in the story was able to help and be that "Guardian Angel" to a relative long ago as well as her Grandmother.
Yes, I believe this book should have been included in the SSYRA since I think the language in this book creates nice imagery. Bauer does a nice job of keeping the story flowing even though it goes back and forth in time. Also, the relationship between the grandmother and granddaughter is sweet.