Robert Silverberg
Especially the stuff he was writing in the 60s and 70s. I would stay away from his Majipoor series.
I especially recommend:
A Time of Changes
Dying Inside
Downward to the Earth
To Live Again
Tower of Glass
Up the Line
The World Inside
The Second Trip
Hawksbill Station
And his masterpiece, and one of the best books ever by an SF author (it isn't EXACTLY SF, but its written with an SF attitude), The Book of Skulls.
If you read any of them, I will envy you.