
During an interstellar war one side develops a language, Babel-17, that can be used as a weapon. Learning it turns one into an unwilling traitor as it alters perception and thought. This is discovered by the starship captain Rydra Wong. She is recruited to discover how the enemy are infiltrating and sabotaging strategic sites.
Babel-17 and Nova are probably both good places for him to start. They are both pretty interesting stories.
Jewel Hinged Jaw is literary criticism and advice for writers https://www.amazon.com/Jewel-Hinged-Jaw-Language-Science-Fic...
His three most pure "science fiction" novels are Babel-17, Trouble on Triton (originally published as Triton), and Nova. They are all very imaginative but grounded in scientific extrapolation.
His Neveryona series of stories and books are set in a medieval frame but are really about modern life. I found them interesting and thought-provoking.
He wrote a short autobiographical piece about some time he spent in a commune called "Heavenly Breakfast" that I found insightful.
He has a number of very good shorts science fiction stories from the 60's and 70's that are very good as well. Two in particular I enjoyed:
"Time Considered as a Helix of Semi-Precious Stones"
"We, in Some Strange Power’s Employ, Move on a Rigorous Line"
are collected in "Driftglass"
I do think PKD improved as a writer in the 1970s, once he kicked his drug habit and slowed down his hyperactive output; from this era, Flow My Tears the Policeman Said, A Scanner Darkly, and the VALIS trilogy all has some beautiful writing and show Dick as a deeper and more mature writer.
As for the rest of the sci-fi genre, there's a lot of talent among his contemporaries. I would especially suggest:
* John Crowley. Widely recognized as actual an Author of real Literature, though only his earliest works could be called science fiction. Start with The Deep and Engine Summer.
* Harlan Ellison. Some of his stories ("I Have No Mouth and I Must Scream", "Jeffty Is Five", etc.) are simply masterfully written.
* Samuel Delaney. He's an acquired taste and certainly gets a bit self-indulgent, but novels like Babel 17 are often beautifully written.
* Ursula LeGuin, e.g. The Left Hand of Darkness.
* Gene Wolfe. Increasingly also recognized as a genius, I also particularly like his short stories. Start with The Fifth Head of Cerberus; graduate with the monumental The Book of the New Sun, which has no equal in or out of the genre.
* J. G. Ballard. A consummate stylist, he eventually drifted away from SF, but his earlic works are fantastic. My favourite is The Crystal World, as well as his numerious short stories, including one of my favourite stories ever [1]).
Of later writers, William Gibson, China Mieville and Iain M. Banks come to mind.
[1] "Report on an Unidentified Space Station": http://geosci.uchicago.edu/~kite/doc/roauss.htm