
A drug dealer of the future periodically moves away from his spaced-out world to become an informer for narcotics agents until he becomes unable to separate his two personalities.
Philip K. Dick: Ubik, Flow My Tears The Policeman Said, A Scanner Darkly, etc. too many to count (many more than twice)
William Gibson: everything at least twice
Neal Stephenson: Snow crash (three times, I think)
Clifford D. Simak: City
H. P. Lovecraft: everything at least twice. There is a French compilation of The Dream-Quest of Unknown Kadath, The Silver Key, and Through the Gates of the Silver Key that I've read countless times. I've used two editions and I'm at my third.
Maurice G. Dantec: Babylon Babies (three times)
Possibly many others.
Philip K. Dick's Ubik, A Scanner Darkly, and Flow My Tears, The Policeman Said.
Clifford D. Simak's City.
Classics.
If you want the intellectual take go with A Canticle for Leibowitz (Miller), Oryx and Crake (Atwood) or Solaris (Lem).
If you want the 60s hard-science rooted societal outlook from an ex-Naval Engineer with strong views on gender roles, it's all about Heinlein - Stranger in a Strange Land, The Moon is a Harsh Mistress, Starship Troopers etc..
If you want something to share with the young adults in your life, or simply some of the finest writing in the contemporary British YA canon, then Philip Pulman's magnificent homage to 'Paradise Lost' - the 'His Dark Materials trilogy' - cannot come more highly recommended. Usually categorised as 'fantasy', and heavily indebted to Milton and Blake, this represents a master-class in parallel-universe world building with its own take on a Steampunk Oxford and a number of other science fiction tropes.
Except that (for e.g.) "Scanner Darkly" is tragic (it's always made me cry) and very funny.
However I do wish that Mercerism (the religion in the book) was included in the film. Maybe someone should attempt to film a more literal adaptation.
Edit: thinking about the difference between the novel and the film makes me think of The Shining and I'm very much in the Kubrick camp - the film being a work of art by itself and so doesn't have to follow the source material.
However, I also really like A Scanner Darkly which is arguably the closest PKD adaptation.