Between several comic versions, more than a dozen sequel novels, and untold reams of fan-fiction, there's plenty of Leia material beyond the original trilogy. These are my favorites.
Splinter of the Mind's Eye by Alan Dean Foster (1978)
Princess Leia is on a diplomatic mission, escorted by Luke Skywalker, when the two are forced to crash on the swamp world of Mimban. There, they discover a crystal which has mysterious powers over the Force. This sequel to Star Wars was published prior to the theatrical release of "The Empire Strikes Back" and caused some continuity problems with its suggestions of romance between Leia and Luke.
Notable Scene: Leia in a lightsaber duel with Vader.
Click Here for Splinter of the Mind's Eye at Amazon.com
Star Wars: Dark Empire by Tom Veitch and Cam Kennedy (1995)
This collection of the Dark Horse comic series is set six years after Endor. Leia and Han are married and have two children. The heirs of Jabba the Hutt have placed a huge price on her head. The Emperor is back from the dead and commands a new fleet of super-weapons. Luke has joined him. Leia walks into the heart of darkness to challenge the Empeor for her brother's soul.
Star Wars Journal: Captive to Evil by Jude Watson (1998)
Princess Leia's first-person account of the events of Star Wars, recorded on a secret datapad hidden in her belt. My favorite portrayal of Leia. A "young adult" novel, but worthwhile reading for Leia fans of any age.
Click Here for Captive to Evil at Amazon.com!
Star Wars Infinities: A New Hope Dark Horse Comics (2001-2002)
What if Luke's torpedo had not blown up the Death Star? In this alternate universe, Leia evacuates Yavin IV just before it is destroyed, only to be quickly recaptured by Vader and Tarkin. She is taken to Coruscant, where the Emperor and Vader try to use her most noble impulses to lure her to the Dark Side.
Click Here for Star Wars: Infinities - A New Hope at Amazon.com
A wide variety of Leia-centered fan fiction, of varying quality.
This page has many beautifully clear screen captures from Star Wars: A New Hope.