These are the two Darkover novels that started me on MZB. Even after reading (and re-reading) all her Darkover novels, these stand out as exceptional. The Spell Sword is a good entrance to the series, as it provides a good introduction to the world through the eyes of an outsider (actually a common theme in this series) and sets up the many tensions that permeate the series -- tensions that arise from having a society in which there are humans and there are non-humans, there are telepaths and there are those that are not, there are lords and there are commoners, there is a relatively metal-less world being exposed to the high-tech star-faring Terran Empire. But all this is background to a fairly simply rescue story that stretches exactly as long as it should to fill a novel less than two hundred pages long.
The Forbidden Tower, which picks up the story almost immediately after the rescue, gets much deeper into character-building and world-building. Darkover almost immediately becomes a much richer world, and the characters that were perhaps a bit stock take on a new depth and break out in unexpected ways. This is a novel of four people who do not quite fit in the places allotted to them -- Callista, as Keeper, has been set aside as special since she was an early teenager and has suffered horribly in the service of what she perceives as the greater good; Ellimir, her twin sister, has always been undervalued because her twin has the lion's share of their talent at telepathy; Damon, who is an incredibly powerful telepath but was exiled from the world he loved because he was a man with a woman's power, and Andrew, the most obvious exile, a Terran on a strange planet who has fallen in love and wants to make it his home. These four individuals come together as family and closer than family and must begin to find a place for themselves on their own terms, rather than their society's. And if this sounds too much like any number of coming-of-age novels, I warn you only that it is not that simple. Each of these people must make hard decisions about who and what they should be and what effect that should and will have on the world around them. It is a powerful book, and I think it is one that will turn open-minded readers on to a fabulous classic science fiction series.