Telling a book lover to choose is a nearby impossible task. I’d just make the whole time capsule and underground bunker library which will hopefully last long past human descendants are a recognizable species.
All Tomorrow’s: this is a very strange yet wonderful SF piece about possible human futures. It’s for hardcore SF fans to read through and an acquired taste, but I think it should be around.
The Lord of the Rings: Arguably the entire fantasy genre started because of this.
1984/ Animal Farm: I’d pick one because it’s the same message really.
Parable Series/ Octavia Butler: because it’s eerily accurate predictions about many of the strains and social ills of her fictional future California are actually true.
Interesting Times/ Terry Pratchett: any of his discworld series but this is my favorite.
To be Taught if Fortunate/ Becky Chambers: I bought it this year and have already read it twice.
Nightfall/Isaac Asimov: because it’s the first SF novel I read and made me love the genre.
Dune/Frank Herbert: it’s not always the easiest read but it has influenced a lot of modern space opera.