Chapter 6 of Star Wars: Heir to the Empire (1991) (EU: 9 ABY) by Timothy Zahn.
Five years after the Empire’s defeat over Endor in Return of the Jedi (1983), and unbeknownst to the New Republic, Imperial Warlord Grand Admiral Thrawn is taking strides in collecting tools and technology left by the late Emperor to aid in his plan to retake territory from — and ultimately destroy — the New Republic.
After a parley with smugglers Talon Karrde and Mara Jade to collect ysalamir, Thrawn weaponises their obscure ability for creating voids in the Force to neutralise Dark Jedi clone Joruus C'baoth and negotiate his allegiance by offering him Leia's unborn twins.
Meanwhile on Coruscant, political infighting seems to be stalling the establishment of structured governance within the Council of the New Republic. Despite personal growing pains, numerous responsibilities, and logistical issues, Han, Leia, and now Luke are assigned a diplomatic mission to Bimmisaari. Luke, it seems, has been requested to join them last minute.
Han and Leia travel with Luke and C-3PO to the planet Bimmisaari on a diplomatic mission to negotiate the terms of the Bimm joining the New Republic. Leaving Chewbacca and R2-D2 on the Millennium Falcon, the four are treated to a brief greeting ceremony. However, before they can arrive at the planetary government, news arrives that the key negotiator for the Bimm has suddenly taken ill.
Luke and Han are invited to inspect relics of the Old Republic while Leia is shown around the market, but a suspicious Han decides to join Leia instead. His suspicions prove true when they find themselves encircled by numerous armed individuals of an alien species they don't recognise.