Look for the Dumarest novels by E. C. Tubbs. These probably had more influence on early Traveller than any other series. It's where the term traveller comes from, for starters, plus CT mainstays like mesh, jack, blade, air rafts, slow drug, low vs high passage -- the list goes on and on. There are over 30 short novels in the series. All of them follow the adventures of Earl Dumarest, a wanderer who's trying to find his way back to Earth, a planet so minor it's been lost from the records ... or could it be so important that it's been excised from public records? He's pursued by the Cyclan, a society of human computers, and often aided by the Brotherhood, a religious order with similarities to the TAS. The novels are from the late 60s/early 70s and filled with two-fisted action. They're somewhat formulaic -- Dumarest arrives on a planet broke and sick from low passage, gets involved in a scheme of some sort (usually driven by whatever local, beautiful woman he hooks up with), discovers that the Cyclan is behind it, foils their plot, and moves on, breaking the woman's heart (assuming she survived). But it's all great fun and the ties to Traveller (although not the Imperium) are abundant. Outside of the specifically trademark-licensed books, you won't find anything with more CT atmosphere.
Other than that, I'd add The Mote in God's Eye, by Larry Niven and Jerry Pournelle, to the lists others have started. It's a first-rate SF story -- some have lauded it as the finest SF novel ever -- and it incorporates a social background for the humans that is driven by interstellar feudal nobility, the same as Traveller.
Steve
