The fabulous Mem Fox wrote this book in Australia in 1984. It includes brilliant illustration by Julie Vivas and was originally published by Kane/Miller Book Publishers. You can find it in any children's literature section, anywhere. This is not too important.
Wilfrid Gordon McDonald Partridge, besides having an exceptionally long name and not being very old either, lives close to an old people's home. His favorite person of all is Miss Nancy Alison Delacourt Cooper, because she has four names just like him. They share secrets and have a generally good time. This is not terribly important, but we're getting there.
One day, Wilfrid overhears his parents talking about how Miss Nancy has lost her memory. The book describes his journey to find out just what a memory is, and how he might bring back Miss Nancy's. This is more important, but not what I need to tell you about this book. Don't worry; I won't spoil the ending.
What you should know about Wilfrid Gordon McDonald Partridge: it is an absolute must-read for any child with a grandparent or other person in their life who might be going through senility--especially if that person suffers from dementia or alzheimer's. Books are a great way for children to cope with changes in their lives, and Fox does a terrific job of putting a softer spin on this one. Alzheimer's is a rough ride for everyone, but it can be particularly scary for children. Not only will stories like these help ease your children's worries, but they promote discussion and help bridge generation gaps.
Some information from my personal copy of Wilfrid Gordon McDonald Partridge.