The 'frieren prequel novel' could be about Frieren's early adventures. It may show how she developed her skills, her first encounters with other important characters in the main series. Maybe it details her life in a time before the main events, like her training days or her first experiences with magic if magic is a part of the Frieren story.
Sure is! Frieren has a dedicated fan base thanks to its engaging plot and well-developed characters. The manga has attracted a lot of attention in the comic world.