According to Japanese Wikipedia, the anime partly covers up to the 3rd volume of the light novel.
AniComiMan (Japanese), a blog that provides spoiler for the light novel prefaced it with
Those who want to know the part that was not included in the anime, because in my opinion there is a big difference, it'd be good to read the original light novel.
A review by Inquisitr (warning: the site included speculation to season 2's material) stated,
Overall, the anime adaptation of the Suka Suka light novel is good and does not sacrifice pacing, although various scenes were rearranged and shortened. Anime usually has to skim over certain details and world building is discarded to fit the source material within 12 episodes. The biggest change from the light novel is that the anime
neglects to reveal the true identity of the First Beast,
but author Kareno probably retconned that spoiler from Volume 1 on purpose since it really didn't make sense to have that revelation so early. A certain beast is not introduced at all in the anime and this character is
critical to Volumes 4 and 5.
There's also little differences. For example, in World End Episode 11 the characters find a girl encased in crystal. [In the light novel...]
Willem activates his "spell vision," an ability never used in the anime, to analyze the curse and the narrative gives more details about the hex's abilities. The book also has a long discussion between him and Grick about the state of the girl and whether it’s possible to resurrect her like what was done with Willem. When Chtholly becomes entranced by the crystal, Willem literally has to plunge his hand into her chest, collapsing her lungs, in order to disrupt her magic flow and return her to normal.
That's just one minor scene, which gives you an idea of how much content the anime skips over. As you can see, it's worth reading the light novels even if you've watched the anime.
Also, the Japanese's impressions on episode 6 (= end of volume 2) are more-or-less: those who have read the light novel were more emotionally moved than those who only watched the anime since the anime skipped a lot/the pace was too quick.