The Library is the name given to a structure present on Installation 04.
The Library's true function is unknown at present. However it is hypothesized to be a collection of all information regarding the Flood known to the Forerunner and also collated cloned DNA sequences. All of this information is contained within the Index, which is housed within the Index Chamber and is used in the process of activating the Halo system and wiping the galaxy clean of sentient life.
The Index is suspended within the core of the Library, out of reach of possible Flood outbreak. The only internal access is via a giant lift that descends from the highest level of the Library to the Index's suspended position.
On Installation 04 the Flood was still able to breach such defenses and infest the Library. This may indicate that the Library is not as secure as the Forerunners had hoped it would be, or that it was initially a containment facility for the Flood. The interior of both Libraries consists of multiple gigantic hallways of geometric design interconnected with one another, finally leading to the Index Room. The large hallways suggests that it was designed to hold large numbers of Forerunners, possibly acting as a backup refugee camp against the Flood.
343 Guilty Spark teleported John-117 from 04's Flood Containment Facility, after deeming him to be a Reclaimer, into the Library of Installation 04 and instructed him to retrieve the index.
It is here that 343 Guilty Spark remarks upon both the nature of the Flood and the Forerunners. Making specific reference to their technological superiority, he wonders why John had brought "such ineffective weapons to combat the Flood." He later tells John-117 that his suit will serve him well as the Flood would try to alter the atmosphere for their own needs (possibly referring to the Flood spores spreading throughout the facility).
The Flood had heavily infested this library in the few short days since their initial breakout, with large groups using the ventilation shafts and access corridors to make their way through the facility. There were Flood in almost every part of the complex, including the Index Chamber itself, and the Sentinel drones and Enforcers assigned to the facility were mostly overwhelmed. By the time John arrived, the Sentinels were reduced to small groups using the shafts to avoid destruction and the Flood had begun terraforming, spreading their spores through the facility by using the ducts.
Toward the end of this Library, John encountered the body of a Marine who was Guilty Spark's previous Reclaimer. This Marine was said to have been so mutilated that the Flood could not possibly infest him. He was identified to be Staff Sergeant Marvin Mobuto.
It seems the Covenant also attempted to access the Library on Installation 04. While John-117 explored the dark bowels of the structure, There were checkpoints throughout the Library, some even had Wraiths, but the Covenant forces at each one fell victim to either John-117, the Flood, or both.
With help from the Sentinels, and motivation from Staff Sergeant Mobuto, John-117 fought his way through three levels of the complex and retrieved the index. The Library's subsequent status and Flood infection is unknown, although it was destroyed with the rest of Alpha Halo when John-117 detonated the Pillar of Autumn's engines. The library is not seen from the outside.
- The foremost structure of the Library is similar to the symbols on the stone slabs on the top floor of the bases in the Halo: Combat Evolved map Blood Gulch. It could mean that the structure of the Library is somehow related to Blood Gulch, or that it is purely a common Forerunner symbol.
- In Halo: Combat Evolved, John-117 gets in the Alpha Halo Library through teleportation by 343 Guilty Spark. In Halo 2, the Arbiter goes through a gondola system to get in the Delta Halo Library, it may be possible that you can get into the Alpha Halo Library as well through a gondola system but it is unknown for sure.