The Deryni books are great sword and sorcery fantasy. What makes them unique is that they're a blend of historical fantasy and high fantasy. What marks a book out as high fantasy is a completely imaginary world with no links to real history--legend maybe, but the ties are tenuous, even when like Tolkien's Middle Earth, Lackey's Valdemir or Pierce's Tortall, they have a pseudo-medieval feel.
This on, the other hand, is Christian Europe--yet not quite. Gwynedd is recognizably Britain--more so than what you see usually see in high fantasy, even if there aren't any real historical parallels to the Haldane dynasty--or the Deryni for that matter, magically talented people who are persecuted by the Church. But more unusually, their "Holy Church" is quite recognizable as the Roman Catholic Church, and the church's beliefs are important to the characters, particularly Bishop Duncan McLain, a Deryni and priest, one of my favorite characters in the series. This isn't like Pullman's His Dark Materials. Kurtz's Holy Church isn't evil, and there are good people within in--and yes, evil ones such as Archbishop Loris. I felt for Jehana, King Kelson's mother, who tries to reconcile her religious convictions with her Deryni heritage.
One thing that also makes this series different is that instead of standalone books with a common backdrop, or closely connected books that proceed chronologically, they tend to be grouped into trilogies. The first, The Deryni Chronicles, focused more on Kelson's supporters, Duke Morgan and Bishop McLain. The focus on the "Histories of King Kelson" are naturally on the young king who came to the throne in that first trilogy. I do like how Kelson is developed here. He's an admirable character by and large, but Kurtz doesn't pull her punches about the more ruthless qualities and actions his position brings out in him. I find this a very enjoyable series with characters I cared about.