Anything that's written by John Varley.
Anything older by Harlan Ellison.
Heinlein's older stuff. His later stuff got ridiculous.
Silverlock author unknown may not be in print any longer, but is a great romp with references to all the great books of the last 1000 years or so. Never stated outright, but if you've read any of the classics, you'll recognize the characters.
The Tarn series, for flat out porn sword/scifi. Actually started as a joke book, but sold so well the author kept writing more of them. Googling Tarl Cabot should get you some links.
Stuff by Larry Niven and/or Jerry Pournelle. Niven's Known Universe short stories, novellas and books were fun. And you have to read The Mote in God's Eye.
Stuff by Iain Banks.