What Larry said.

Priest is a school of the 1970s New Wave of SF, which attempted to introduce literary ideas to science fiction. His early novels were a lot more traditional SF, with INVERTED WORLD set inside a Big Dumb Object and THE SPACE MACHINE being a sequel to THE TIME MACHINE (as well as amusingly postulating that THE TIME MACHINE and WAR OF THE WORLDS are set in the same universe).

Priest also garnered a lot of notoriety in the 1970s when he published THE LAST DEADLOSS VISIONS, an essay which basically accused Harlan Ellison of incompetence and dishonesty in the much-delayed publication of THE LAST DANGEROUS VISIONS, an anthology Ellison was supposed to publish in the early 1970s but remains MIA even today. Given Ellison's 'untouchable' status in SF at the time, it caused an enormous stir.

Priest changed tack with A DREAM OF WESSEX in 1977, which was a very early example of VR fiction (and acted as a precursor of cyberpunk). Priest introduced his ideas of shifting realities in this book which would reappear later: the idea of the reality the character is experiencing being somehow malleable and prone to change, sometimes something the characters are aware of (and confused by) or something only the reader is. This garnered the term 'The Priest Effect'.

THE AFFIRMATION (1983) introduced the Dream Archipelago, a massive chain of islands on a different planet which is the closest thing Priest has to a signature setting. This planet appears to be an alternate-reality version of Earth. Characters flit back and forth between the 'real' Earth and this alternate world in various different ways.

THE PRESTIGE (1995) is Priest's most approachable novel - out of his recent stuff anyway - as it doesn't feature the complete collapse of reality or something equally huge. Instead it's the story of two magicians feuding with a framing story (missing from the movie) set in the present day. It's very good.

THE SEPARATION (2002) may be his best novel. It's the story of twin brothers in WWII, one serving as a fighter pilot and the other as an ambulance driver, and also about the people in the present who are researching their story. The novel moves between several different timelines resulting from the decisions made by the brothers at the time, which is rather confusing as Priest doesn't give you any warning of when he shifts timelines. When you 'get' it, it's quite impressive.