The top selling light novel is Demon Slayer. Over 1 million copies sold you can see why.  The story has the characters in modern times in high school. As far as I know there are no ties to how the series end.

This list covers sales surveyed from November 18, 2019 to May 17, 2020.

Rank	Title	Author	Estimated Sales
1	Demon Slayer: Kimetsu no Yaiba Novelization	Original Story: Koyoharu Gotouge/Novel: Aya Yajima	1,199,863
2	Sword Art Online	Writer: Reki Kawahara/Illustration: abec	377,493
3	That Time I Got Reincarnated as a Slime	Writer: Fuse/Illustration: Mitz Vah	327,191
4	Kusuriya no Hitorigoto	Writer: Hyūnganatsu /Illustration: Kashiwa Miyako	317,749
5	Overlord	Writer: Kugane Maruyama/Illustration: so-bin	264,875
6	Re:ZERO -Starting Life in Another World-	Writer: Tappei Nagatsuki/Illustration: Shinichirou Otsuka	264,458
7	My Teen Romantic Comedy SNAFU	Writer: Wataru Watari/Illustration: Ponkan8	256,704
8	Konosuba – God’s Blessing on This Wonderful World!	Writer: Natsume Akatsuki/Illustration: Kurone Mishima	223,657
9	Seishun Buta Yarō Series (Rascal Does Not Dream of Bunny Girl Senpai)	Writer: Hajime Kamoshida/Illustration: Keeji Mizoguchi	173,391
10	Classroom of the Elite	Writer: Syohgo Kinugasa/Illustration: Shunsaku Tomose	150,689