An omnibus of The Spell Sword (1974) + The Forbidden Tower (1977)

These take place in the middle of the series - about 30 years before Star of Danger, which comes a generation before The Bloody Sun. A large portion of the Matrix-technology of Darkover is expained and worked out in these books. The Forbidden Tower is a direct sequel to The Spell Sword, involving most of the same characters, beginning less than a year after the events of that novel.

Bradley wrote books in her "Darkover" series throughout her career - this was a particulary rich world and backstory which brought out her best work. As with Norton's "Witch World" or McCaffrey's "Pern" sequences, I feel there is a core of best works which should be read in preference or before other parts of the series - these are from the top half of that list. Don't try to read the series by internal chronology (definitely don't start with Darkover Landfall.) This set would be a good place to start reading the series.

Both books were paperback originals, which were later reprinted in hardcover by Gregg Press (who tended to very small print runs of a few hundred or so.) This hardcover omnibus from SFBC came out a few years after the DAW paperback omnibus of these - much to my surprise and delight. After decades of searching for reasonably priced hardovers (and failing to find any), I filled in my own collection with another copy of this.

The Spell Sword (copyright 1974.) This is one of my favorites by Bradley, and makes my personal "top 100" list of best SF books.

Andrew Carr was a Terran, who had diverted from his plans and stayed on Cottman IV (Darkover to the natives) after a vision shown to him by a fortune-teller. He takes a job with Mapping and Exploring, and he is the only survivor when his plane crashes in the Hellers. Much to his surprise, he is aided by the girl of his dreams, who saves his life twice by appearing to him and directing him to shelter.

Damon Ridenow is one of the Comyn nobility, summoned to Armida by the Lady Ellemir after her twin sister Callista was abducted by Catmen. Damon is ambushed by Catmen on his way, and barely escapes with his life because he has a starstone and is Tower-trained, able to see the otherwise-invisible Catmen. Callista and Ellemir are the daughters of Dom Esteban Lanart, Lord Alton - ruler of the Alton Domain, and high in the Comyn Council. Callista was raised to be a Keeper in one of the Towers where the Darkovans used their matrix technology. Her sister had less talent, but being Comyn, is still a minor telepath. Damon does what he can to restore order at Armida, and tries to search the Overworld for Callista, but apparently she is hidden or blocked to him somehow.

And then Andrew Carr shows up on the doorstep, directed there by Callista. There is a nice bit of culture clash as Carr, who has formed a close bond with Callista, offends the Darkovans who feel he doesn't display the proper reverence for the virginal Keeper. Dom Esteban returns, crippled in a fight with Catmen who ambushed his party. He was the only reason that any survived - one of the greatest swordsmen in the land, fighting with his eyes closed. Now Damon Ridenow must assemble the available pieces to make a rescue party for Callista. Normally a shy and reserved person, told he was strong enough to be a Keeper if men were permitted that station - he has to call up somehow the resolve and will to do what is needed. Alternating viewpoints between Andrew Carr and Damon Ridenow, this is a perfectly-told tale, full of action, color, and romance. A good portion of the Matrix-technology of Darkover is expained and worked out here.

The Forbidden Tower - a direct sequel to The Spell Sword, involving most of the same characters, beginning less than a year after the events of that novel.

The 4 main players from the previous novel: Andrew Carr, a Terran; Callista, the girl of his dreams (raised to be a Keeper in one of the Towers where the Darkovans used their matrix technology); her twin sister the Lady Ellemir; and her fiancee Damon Ridenow - all had been bound together when they had to rescue Callista after she was abducted by Catmen. Damon had been told he was strong enough to be a Keeper if men were permitted that station. The course of events leads this group to decide that they could and should create their own Circle - forming their own Tower in defiance of repressive teachings.

Another of several Darkover books bound together by the appearance/presence of Leonie Hastur. This was a Hugo-nominee (for "best novel") in 1978.