Schismatrix by Bruce Sterling
This was the first science fiction book which blew my mind.
I was sixteen years old and reading it in a grungy motel on the beach in Samoa where my family stopped for a holiday in the process of moving to New Zealand. What I've always loved about Bruce Sterling is his characters, he writes character driven, grungy cyberpunk. It's full of meat and emotions and all the failures of humanity. There is no holy grail, only humanity driven and changed by the relentless pursuit of technology.
I've lost count of the number of times I've read, bought, or loaned this book.
In my top three science fiction books of all time.