Isekai no Seikishi Monogatari

Written by the original creator Masaki Kajishima (with Wadi Horiguchi), the Light Novel series began publication after the anime concluded.
The Light Novel covers the same events depicted in the anime, but it does so with vastly more detail. You get deeper character thoughts, more intricate world-building explanations, expanded political context, additional scenes, and more fleshed-out backstories that the anime had to condense or skip for time and pacing.