DD7 is enjoying Word Snoop (thanks jojo!). It's a small hardback written to what looks to me about a 4th-grade reading level which works out very well, with short chapters so they can pick it up anytime and get something out of it. Even though it's the American edition it uses British (Australian?) spelling and DD immediately asked me why they were spelling 'color' as 'colour', etc.

Books like this produce those "ah hah" moments and looks of delight in DD's face, so they are invaluable. Just need to sniff out more now...