At the end of Solo (2018), after having a messy, contentious relationship with Han, Lando abandons him in the Millennium Falcon. Han returns to win the ship and then departs.
About 13 years later, in The Empire Strikes Back (1980) Han turns to Lando for help, believing that he'll get it and trusting Lando to a degree.
Obviously, this implies they had some sort of relationship (or at least interacted) in that stretch of time.
Are there any books or other sources that mention them working together or anything in this time frame?