I have fond memories of Watership Down. I was among the generation of children who first saw the 1978 animated adaptation of the book. I read the novel itself eight years later, as a senior in high school. Watership Down is one of those great books that everyone should read at least once, and I can't recommend it enough.
While the death of a 96-year-old from natural causes cannot rightly be called a tragedy (let's face it--we all have to go sometime) it is still a loss, especially in this case.
We can be thankful that Richard Adams left Watership Down (and a handful of other fine books) for the enrichment of succeeding generations.