This is such sad news. I loved her books so much. I think I found her because I watched the adaptation of Who, Sir? Me, Sir? when it was on tv, then got the book, and then wanted to read more of hers.
I loved the Pennington books, but the ones I read over and over again were the Flambards ones. Well, technically Flambards and The Edge of the Cloud and then the first chapter or so of Flambards in Summer.
The Edge of the Cloud was one of my favourite books as a teen as I was so absorbed by Will and Christina's relationship.