Is this like another plane we ascend to or is it a gigantic library?
Both, although we skip the Akashic plane on the way back to the Tao.
It's a repository of everyone's experience, as they experienced it, not a record of what actually happened from some absolute viewpoint. Interpreting it from the physical plane is very, very difficult. Michael is able to synthesize multiple viewpoints and track things down through many, many different people's experience. This is frankly impossible for a physical plane person -- there's simply not enough processing power available.