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.
Regret is a level that I like almost in spite of myself. It pulls off an impressive sleight of hand, managing to create a fair impression of size and continuity in a mission that contains relatively little playable surface (comparatively) with areas that are linked only by pieces of moving geometry that the player has only rudimentary control over.