I'd say I'd prioritize the ones that have the most impact and usefulness on society. That means:
- all those self-help motivational books gotta go
- the books written by politicians for the sake of campaigns have no place here
- most school & college textbooks come and go all the time (fick you, Pearson) so I wouldn't include them, but there are all time classics that are worth saving
- all the religious scriptures (e.g. holy bible, quran, tripettaka) gotta stay, no matter how bullshit you and I might think
- classical philosophy stays, probably some modern ones as well (but I don't know where to draw the line)
- books about facts (i.e. science & history) documents stays, but not all of them; this is a hard one, some rigorous verification
- technical guidebooks (e.g. construction, gardening, electronic, software, machinery, chemistry, metalurgy) gotta stay, but needs rigorous verification, also technology changes all the time, so most of the outdated ones might need to go (saving some for historical purpose)
- fiction is the hardest, but generally the all time classic (1984, anyone?) as well as the really popular ones (sadly includes Twilight) stays
Whew... This is way more work than I initially anticipated.