Just finished season 1 of Andor. Absolutely loved it and want to know if there are any books or media that tells what will happen to Cassian between now and Rogue One.
So far there are relatively few non-Andor appearances of Cassian Andor in material taking place before Rogue One.
Cassian & K-2SO Special (2—0 BBY)
Rogue One - Cassian & K-2SO Special 1 is a standalone comic issue taking place between 2 and 0 BBY, and shows how Cassian and K-2SO met. It came out in August 2017 and takes place after the founding of the Rebel Alliance, which happened in 2 BBY.
Secrets of the Empire (2—0 BBY)
Secrets of the Empire is a "VR experience" formerly available at The Void locations. The players are given a mission by Cassian to infiltrate Darth Vader's castle on Mustafar. Cassian introduces himself as "Captain Cassian Andor of Rebel Intelligence", indicating that it takes place after the founding of the Rebel Alliance in 2 BBY. It came out in December 2017.
The Mighty Chewbacca in the Forest of Fear! (0 BBY)
Cassian also appears in the junior novel The Mighty Chewbacca in the Forest of Fear! I haven't read this one, but Google Books substantiates Wookieepedia's claim that Cassian makes an appearance:
"Captain Andor, why is it important?"
"That's classified information, and these are the Emperor's errand boys," said Cassian. "I think I'll keep that to myself."