I really started loving the Library once I saw
the speedruns for it. If there was ever a level that encouraged you to run and gun and make use of map awareness on top of the sandbox, the Library was it. It was oddly prophetic regarding horde / firefight-style modes in that it had a lot of "hold this position while I unlock the door" sections, but those can actually be skipped. Fun fact: you can actually cheese two of the floors (2nd and 4th) by going into the main "inner rim" area facing the Index instead of entering the combat halls immediately after each elevator and crouch-jump over some struts straight to the next elevators. You can also bypass 99% of the door-unlock firefights by grenade-jumping onto the lower accent of each door, allowing you to squeeze through the remaining opening rather than having to wait for the whole thing to open. The only one you can't avoid is the final one, which you can still cheat by hiding off in a corner and shotgunning the two or three infection forms that might spot you.
Plus, on a more sandbox level, the Library is essentially a Doom chapter on steroids. It's an acquired taste, for sure, but it's awesome seeing it sped through.