When a book or series of books is part of what you've built your soul upon, it doesn't seem unreasonable to me to grieve when you learn that there will be no more. (Heck, I kind of feel that way about Dorothy Sayers, and I'm pretty sure she died before I was born. At least Diana gave us a big and varied corpus of books.)

As for grieving for her as a person, I feel a little presumptuous too, not having known her. But then on the other hand, there are people who, by all accounts, make the world richer for their existence in it, So being sad that the world has been impoverished by that much (while simultaneously glad that they did exist, in the same way you feel at the death of anyone you care about) does make sense. Diana Wynne Jones sounds like one of those people to me.