Cortana, H3. And the library is actually, and always was (even day 1 in 2001) my favorite halo level. Not sure why everyone has so much distaste for it.
I do completely agree on your point that The Library is a relatively fun mission in Halo CE. In addition to that, it's also quite relaxing due to the fact that the level is relatively predictable and you aren't really forced to switch weapon for different combats.
Another additional bonus with The Library is that it's one of the easiest missions in Halo CE due to it being so predictable and due to the fact that you aren't forced to deal with Elites or Rocket Flood at short range.
are you even kidding??? the Library is FILLED with rocket flood!?!?! it is one of the HARDEST Halo CE levels EVER! on of the Hardest HALO lvls every. only topped by grave mind from Halo 2.
If I remember correctly, there do only exist 3 different Rocket throughout the mission. The first is in the 2nd flood elevator shaft, the second is the in the 2nd floor in the main corridor directly after the small underpass and the 3rd is the the third floor directly after the door to the elevator shaft opens. Out of these 3 Rocket Flood, the first can easily be skipped and the others can be taken with the the Pistol from a safe distance as long as you are prepared for them.
My main strategy for The Library is just to know the mission and know which sections that can be skipped with relative ease and which enemies that should taken.