Hello,
I just downloaded the Lonely Planet England ebook (thanks for the tip Mardee). They seem to be really excited about Castle Howard and recommend it in the "Our Picks" section at the beginning of the book, the introductions to "Beyond York" section and then more detail in the "Beyond York" section. From some forum comments and probably some other research I've done, I decided Castle Howard was not something I needed to see. I'm wondering if there's something I'm missing.