Heritage Auctions has some wonderful high-res versions of Keith Birdsong cover illustrations from several early 90s Trek novels up at their site. It's great if you want to see the original art without the trade dress. There's stuff from TOS, TNG, DS9, VOY, and New Frontier novels, including Doctor's Orders, Legacy, Traitor Winds, Recovery, Federation, Best Destiny, Vendetta, Reunion, The Captain's Daughter, Relics, and the first book of the Khan Trilogy. I particularly liked seeing the original cover art of Enemy Unseen, back when it was set in the era of "Where No Man Has Gone Before" instead of TMP.
If you're interested, you can see the art over here.
If you're interested, you can see the art over here.