These are some oldies from the 60s-70s that certainly shaped the thinking at GDW.
James H. Schmitz His Hub is almost as big as Third Imperium, with megacorps, criminsal consortiums and star-spanning institutes and Societies. Pirate gangs who think nothing of snuffing a few thousand people to cover their escape. I don't run psionics, but his Telzey Amberdon stories show how to do it right.
Alexei Panshin (At least his novels
Star Well,
Masque World, and
The Thurb Revolution) His Anthony Villiers, Viscount Charteris is the epitome of the Imperial noble, witty, urbane, unfailingly polite. And the stories have a light-hearted tone, until you realize the casual acceptance of assasination, boot-leg organs of uncertain origins, and a murderous
code duello. I think the LBB
76 Patrons stat'ed him.
http://www.strangehorizons.com/2003/20030324/villiers.shtml
A. Bertram Chandler The John Grimes novels (tracing the career of that Naval officer, but they read as more Merchant than Navy) are usually remembered. Chandler spent a career at sea as a merchant marine officer and master mariner and his novels are the best source to understand Merchant characters. He was also Australian, which adds an interesting flavor.
http://www.bertramchandler.com/
Schmitz is currently being reprinted, good luck finding Panshin's Villiers novels or anything by Chandler. I haunted used book stores for my copies.
Gordon Dickinson, Especially his Dorsai novels (I particularly liked
Tactics of Mistake). These are about a planet of Mercenary soldiers
Jerry Pournelle A little later. His Falkenbergs Mercenary Legion stories are about the bast out there.
Larry Niven and he wrote
Mote in God's Eye is one of the classic first-contact stories.
Nerwer stuff ... I rather like
Robert Frezza (his McLendon's Syndrome novels are very Traveller-esque).
Robert Lynn Asprin has done some Travelleresqus stuff. The Phules Compsny stories are fun, but his novel
Tanbu puts a new spin on Space Piracy