Though they have been featured in a number of books, they twins have a book series dedicated to them and their adventures in Jedi training called the "Young Jedi Knights Series".

Introduced as Fetuses in the expanded universe Novel, Heir to the Empire, Jacen and Jaina Solo were expected for great things even before they were born. Even without the name, the Skywalker legacy follows them and thier younger brother Anakin Solo (namesake of his Grandfather). Because of the Legacy to which they were born, thier youth was filled with being hidden away for thier protection. Unlike there mother and Uncle Luke, there's no disputing these two are twins. Physically, the only way for them to have been looked more alike is to have been born the same gender.  Despite this, they couldn't have moe individual personalities.
 

Han Solo and Leia Organa Solo's first daughter, born on Coruscant. She is five minutes older than her twin brother, Jacen, and is equally strong with the Force. She has certain mechanical abilities, much like her father, but not as strong as her brother, Anakin. When she and Jacen attend Luke Skywalker's Jedi Academy on Yavin 4, at the age of 14, she collects all sorts of old technology from the original Alliance base, using some of it to create cages for Jacen's exotic collection of creatures. Others she keeps to see if she can repair them. Jaina created a lightsaber of violet light while studying at Luke Skywalker's Jedi Academy on Yavin 4. She later trained directly with Mara Jade, after Mara's marriage to Luke, when Luke began moving the new Jedi Knights back to the pre-Empire style of teaching. A real student of the Force, Jaina found it hard to be around her mother when she tried to use the Force. Jaina felt that Leia played with her Force skills . During the Yuuzhan Vong invasion of the galaxy, Jaina put her skills with the Force to work as a member of Rogue Squadron, flying an X-Wing into battle against the aliens. She survived a number of battles, but was forced to eject from her fighter during the defense of Kalarba. She was caught in the explosion of the Champion, and suffered blinding damage to her eyes which left her with fuzzy vision for several weeks. She was shipped off to Duro, to make room for more desparate injuries, and helped her father in the evacuation of Settlement Thirty-two. Over time, her eyesight returned, and Jaina longed to get back into the cockpit. However, her status as a Jedi Knight forced Gavin Darklighter to place her on extended leave, in order to appease the politicians of the Senate. Jaina agreed to be part of the Jedi strike team which was dispatched to Myrkr to eliminate the voxyn queen, along with both of her brothers. She was unprepared for the losses the Jedi took just to infiltrate the Baanu Rass, and was was overcome when Anakin was killed aboard the worldship. She openly blamed Jacen for Anakin's death, claiming that Jacen left him to die. In her anger over the loss of Anakin and her blame of Jacen, Jaina turned to the Dark Side of the Force. She conjured up an intense storm of Force lightning when a Yuuzhan Vong nearly killed her, utterly destroying the alien attacker without remorse. After finally destroying the voxyn queen, the Jedi were forced to flee the worldship in order to return to Eclipse. Jacen, however, had been captured trough double-crossing. Jaina was forced to leave him , but she vowed to rescue her brother, despite her earlier animosity toward him.

Han Solo and Leia Organa Solo's first son, born on Coruscant. As is his twin sister, Jaina, he is very strong in the force. Jacen has the special ability to communicate with animals. When he and his sister turn 14, they attend Luke Skywalker's Jedi Academy on Yavin 4. While there, Jacen developed an extensive collection of the jungle moon's exotic fauna, many of which escape the cages built for them by Jacen's sister, Jaina. When he finally constructed his own lightsaber, Jacen chose focusing stones which produced a glowing, emerald green blade. He later trained directly with Luke himself, as Luke began moving the new Jedi Knights back to the pre-Empire style of teaching. Luke also trained Jacen's brother, Anakin, although both brothers had different ideas on what the future of the Jedi should be. Jacen agreed with Luke, that the formation of a new Jedi Council would help bring the vigilantes under control. Jacen also believed more in the spiritual aspects of the Force. They fought often, verbally and with lightsabers, both claiming their intense sessions were simply training. He traveled with his younger brother, Anakin, to Centerpoint Station, just before the Battle of Fondor, but urged Anakin not to use the Force to fire it. When Thrackan Sal-Solo fired it instead, and nearly destroyed the combined Republic and Hapan fleets, Jacen felt himself lost in his struggle to understand the Force. After having an intense vision of the galaxy, teetering on a single balance point that he alone controlled, Jacen tried to stop using the Force altogether. His family tried to bring him out of this mental state, but Jacen doggedly refused. However, when his mother was threated by the Yuuzhan Vong on Duro, Jacen realized that he had reached the balance point in his vision. Drawing on the Force, he managed to escape with his mother and get her to safety. Shortly afterward, Jacen and his siblings joined the Jedi strike team which was sent ot Myrkr to locate and destroy the voxyn queen. When Anakin was killed aboard the Baanu Rass, Jacen found that his sister blamed him for Anakin's death. Eventually, the Jedi were able to defeat the voxyn queen, but Jacen was captured when Vergere betrayed him to the Yuuzhan Vong. Despite her changing allegiance, Vergere provided Jacen with several of her healing "tears," to help prepare him for the torture he was about to receive after his capture.