Juliana just let me know that she's heard through the blog-grapevine some sad news about Diana Wynne Jones, which is that she has recently opted to cease chemotherapy (http://www.locusmag.com/News/2010/06/diana-wynne-jones-health-update/). Here's a blog post about sending fan mail, which now would indeed be a good time to do:
I sent my own little heartfelt note today. However, it seems to me that anytime is a good time to send fanmail to Diana Wynne Jones!
In my note, I pointed to The Lives of Christopher Chant and Dark Lord of Derkholm as my two personal favorites, the former because it details a situation in which a child has to learn to be skeptical of adult motives and the latter because it offers metal-level commentary on the tropes of fantasy while also being emotionally engaging and having a rousing good plot. I also love having almost-too-many characters to keep track of, as long as the author gives us good reason to care about the characters. Which Jones always does.