Here is a link to his author page on LibraryThing and one to his name page on Worldcat Identities. There are plenty of obits. I've listed a few at bottom.
He was 83; died from cancer a couple of weeks after the death of his wife, Janine Denni. They had been married 60 years and lived in Drexel Hill where he had been born and raised.
The New York Times quotes his acceptance speech on being awarded the Newbery Medal in 1969: "In whatever guise — our own daily nightmares of war, intolerance, inhumanity; or the struggles of an Assistant Pig-Keeper against the Lord of Death — the problems are agonizingly familiar. And an openness to compassion, love and mercy is as essential to us here and now as it is to any inhabitant of an imaginary kingdom."
Tim Burke enlarges on this theme in a tribute at Easily Distracted. Says he, "To me, the books were valuable not just as a story of swords and sorcery or even of the journey from childhood to adulthood, but also as an exploration of what it means to make moral choices. ... I think it is utterly counterproductive to teach morals by diktat and repetition. Any story for children that has a single or obvious moral teaching is a story begging to be ignored, subverted or rejected. The Prydain books explored morality as it is lived, even for children, in difficult choices, in painfully-won wisdom, from the inside of consciousness rather than the outside infrastructure of social life. ... The main characters are not noble by fiat.... One of the incidents that made the biggest impact on me as a boy was when Taran is compelled to accept the possibility that his lost father is not of noble birth, but a shepherd, and the shameful feelings he struggles with as a result. Characters die, characters suffer. When they come to a moral decision, you’re taken along with them inside the process of experience and reason that brings them to that moment. ... This is not about saying that everyone’s right, that all choices are ok. These are the kinds of fictions that take children (and adults) through the process of moral reasoning and make them relive ethical choices as painful, difficult and not blandly equanimous."
Some obits: NYT, CBC, LA Times, Boston Globe, NY Mag Washington Post.