This is gonna be a short answer because i I have been doing library for a very long time, you time. You just cant know wich which ones you have alredy killed unless you have killed the final boss, the already killed unless you have killed the final boss. The final boss is the only one that makes a change because it gives you an achievement first time you complete it "library liberator"
if i liberator".
If I may do a suggestion that might help you, start you. Start with the hardest one and test, you test. You cant get into the room with the lever unless you have killed all the bossesbosses.