First of all, let's get this straight. This level isn't called The Library, but it might as well be. You don't actually reach the structure on Installation 05 that houses the Index, the one that's called The Library by Tartarus and Commander Keyes, until the very end of the next level, Quarantine Zone. However, that level is a mix of indoor and outdoor environments with a bunch of vehicles thrown in and friendly Covenant troops alongside while you battle both the Flood and the installation's defensive systems.
You first see those systems in Sacred Icon: tiny, glowing gossamer versions of Sentinels that flit from place to place, seemingly to try and repair the horrible mess that part of the installation has become; the usual Sentinels and others with more powerful, blue sentinel beams; and the Enforcers, huge floating juggernauts (vaguely resembling the units that shared that name from the Marathon series) that rapidly fire needler-like rounds from the front, a barrage of missiles from the top, and can crush vehicles between two powerful legs if they get close enough.
The stucture of Sacred Icon is both derivative of The Library and indicative of a conscious attempt to address some of the complaints players had about it: that it was dark, repetetive, didn't feature a wide enough array of enemies, and takes place entirely indoors and without any vehicles. Some of these complaints were addressed in the Quarantine Zone level, which as I've said brings the final approach to the Library outside for a bit, with a chance to use the Spectre and the Wraith while watching the Flood perform their new tricks, barrelling at you in Warthogs and Ghosts. (The animations of the Flood driving vehicles while standing upright on the seats are also quite amusing.)
In Halo's Library, you were teleported to the bottom of a shaft that contained the Index and told by your guide, 343 Guilty Spark, that it was necssary to ascend to the top of the shaft to retrieve the Index. The first question any player would naturally ask himself is why GS couldn't just teleport you to the top of the shaft in the first place.
After you've played through the level, winding your way through dark, repetitive corridors with no allies, fighting only Flood combat, carrier and infection forms, with no Covenant and no Marine allies, only the garrulous Guilty Spark and his silent Sentinel cohorts, you'll be left wondering what Bungie was thinking at all; there's almost no variation in the encounters, save for one point where a door fails to open and you're stuck in a narrow space as Flood pour in all around you.
Sacred Icon addresses nearly all of these concerns. Many areas are appropriately dark: it's understandable that Bungie doesn't really want the Flood associated with sweetness and light. Even Two Betrayals, the open-air snow level that plays as Assault on the Control Room but in reverse and with Flood, is noticeably darker than the previous iteration of the same environment. That trend continues in this level.
Unlike the windowless darkness of the Library, there are many locations in Sacred Icon where the structure you're traveling through-- called the Wall-- is open on one side, allowing you to look out over a wasted snowscape that looks like some kind of twisted evil Christmas postcard from a parallel universe. It also allows the level's collection of Sentinels and Enforcers to take aim at the Flood-- and, incidentally, at you and however many of your Grunt and Jackal buddies that manage to survive to back you up.
Where you traveled upwards in the Library, in the Wall you start at the top and must work your way down. The Wall is designed for the travel of Sentinels, and indeed seems to be a place where they are produced; the entire level is strewn with doors where Sentinels will enter. The production of Sentinels is preceded by an audio cue, and you can destroy the entrances so they no longer function.
Although Sentinels are annoying-- unlike the Library level where they were the Chief's allies against the Flood, here there is no love lost between them and the Arbiter-- you may want to consider leaving them alone for a bit at the beginning of each encounter. They will help soften up the Flood in each area, and destroyed Sentinels will drop Sentinel Beam weapons that are at least as effective against Flood as human weapons are, and more effective than most Covenant plasma weapons-- luckily you'll find few of those here, but there are Carbines and Needlers around. You'll also find a healthy supply of human weapons including SMGs, Battle Rifles, pistols and even a few shotguns, although ammo is sometimes scarce.