The Dragonriders of Pern

It is an open debate whether to shelve these books in fantasy or science fiction.

The Case for Fantasy

The milieu is low-tech, the social structure is feudal, and an elite 'knighthood' are culled from the population to become the Dragonriders, who form mental bonds with giant telepathic dragons, who protect the planet from the scourge of the falling 'thread', that radioactive-like ecological menance that periodically falls from the sky when the Red Star passes too closely to Pern.

The Case for Science Fiction

Throughout the series, McCaffrey puts scientific backstory behind her world. Pern was originally an earth colony that lost contact with the home world; dragons were selectively bred to their huge size and usefulness from tiny winged lizards, and as time goes on, the dragonriders uncover more and more about their technological past.

I file the stories mentally under 'fantasy' because... well, telepathic dragons...

The Dragonriders and Their World

The first Dragonriders of Pern book, Dragonflight, was published in 1968. It sets the stage for all later books. Pern is a feudal society, with Holds (estates) ruled by hold-keepers, rather like medieval Barons. The Weyrs are fortresses that protect the Holds, homes to the dragons and their riders, and the Holds pay tribute of money and supplies to what is essentially their peace-keeping force as well as their defense against the extra-terrestrial threat of the Thread.

The Red Star is a wandering planet in Pern's star system, and thread is an indigenous form of Red Star 'fauna' that, when the two planets are close enough, tries to migrate from one planet to the other. If allowed to land and burrow, it will make the place it claims uninhabitable, killing all other vegetation. Thread that burrows must be burned out.

Thread falls like rain, but can be burned in the air by the flaming breath of the giant dragons. (Dragons must chew firestone before their breath will burn.)

Dragons also have the seemingly magical ability to travel instantaneously from one part of the world to another, by going 'between', apparently a non-dimension that connects all parts of space together. If the dragon is carrying a rider, the rider, and everything else the dragon carries, travels 'between' and out again with him.

Dragonflight

When Dragonflight begins, Pern has gone through an unusual period, during which, due to the vagueries of its orbit, the Red Star has not passed close enough to Pern to drop thread for some time (some hundreds of years). The non-Weyr population has become used to life without thread and resent the tribute they must pay to keep the Weyrs living in luxury to fight a threat that may not exist any more.

The Weyrs, meanwhile, have dwindled; now only one Weyr remains operational, with few dragons and dragonriders available to protect the planet. And the Red Star is growing bigger in the sky every night.

Dragonflight follows the story of Lessa, a young noblewoman, as she fights first to regain control of her family's hold, and then, after she 'impresses' (or telepathically binds) a golden Queen Dragon, as she fights to save Pern from the return of the Thread, with only one Weyr at her command and a whole planet to cover.