The Dome device was a repository of information created by the Alliance of Four Great Races during the height of their cooperation.
The Ancients, the Asgard, the Nox, and the Furlings chose to record their information in a "universal language" by reducing the method of communication to the most basic form in the universe: the elements; whether the elements represented words or letters is still unknown. The Library was stored in a pedastel bearing a marked resemblance to a DHD, which projected the Library into the room when activated. The only Library discovered thus far was at Heliopolis, and was stored in the most structurally secure room in the castle. The Library's status following the sinking of the Stargate is unknown, but it may have survived as it is armoured enough to protect against Staff weapon blasts. (SG1: "The Torment of Tantalus")