Philip K Dick, "A Scanner, Darkly".
Dick's dialogue was often clunky...paradoxically its clunkiness gave it a certain veracity. His themes usually revolve in some way around identity, humanity - but what makes his (non-juvenile) stuff special above almost all other SF, why it transcends any boundary, is that the people and the society he depicts, in whatever book or story, are so rich. Bits creep into most of his best novels that would probably be called "magical realism" if written by some South American author - but somehow that's not the right expression for it. There are always conceptual flipflops that somehow haunt you, too - like when the main character in A Scanner, a deep-undercover narcotics cop, is said to be "posing as a narc."
Apparently someone asked Dick whether he was a character in this novel, (ostensibly it's about Californian drug culture/law enforcement of the near future, it's dedicated to friends & acquaintances that died in various ways from drug addiction) and he said, "No - I am the novel."
His fiction has off-the-wall concepts sometimes, but it usually has very prosaic, suburban-like settings, and people with prosaic (at least to them) nonentity lives, so it connects very directly. (Like the neighbour woman in "a Scanner" who calls the main characters to get rid of a big beetle in her house. When they scoop it up and put it out, she says "Well, if I knew it was harmless, I'd have killed it myself!")
Funnily enough, his "scramble suit" in the novel, which I think the predator's clothing in the Predator movies was based on - the US military are very close now to developing a working one.
QUite a few films of his stuff have been made, but unfortunately, they often murder the very essence of his stories - Blade Runner (not too bad) Minority Report, Total Recall etc.
I had read every science fiction book in my local library by the time I was about 13, and there are loads written since that I love. China Meiville's "Perdido Street Station" is one of the more recent ones that stands out. (I can't be arsed with the finer-and-finer slicing of SF into sub-sub genres by critics and booksellers: who cares if it's labelled "fantasy" or "steampunk" or whatever.)