Going through the book Serpent Kingdoms I have seen several references to a being called the World Serpent. From what I have gathered it was some kind of over-deity from which the deities of the Scaled Ones came, and has been around since AD&D. Is there more to the World Serpent than this, and if so in which books can I read more about it? Note that I am looking for pretty much any source that predates 4e because of the changed role the World Serpent and the Primal Spirits have in that edition.
The World Serpent is directly borrowed from real-world Norse mythology, in which the World Serpent Jörmungandr encircles the world and is destined to end it.
The Serpent Kingdoms is a Forgotten Realms book, and like most Realms mythology, liberally borrowing from real-world mythology is a deliberate world-building choice to imply that our real world and the Realms are somehow connected.
Although the World Serpent has shown up in D&D since as early as AD&D (if not earlier), it's truth has never been nailed down. (Gods in pre-WotC D&D not attached to a certain setting are not canon, they're raw material for the DM to world-build with.) Despite the World Serpent appearing in a WotC setting book, it's not really nailed down either—the World Serpent is part of the various Serpent Folk mythology, and isn't really solidly defined as "truth" elsewhere, leaving its truth in doubt.
So there isn't really a "what exactly is it" outside that book. The truth of the matter is left up to the DM to play with, just like blank lands on the map of the Realms. For solid inspiration, you should go back to the source that Ed Greewood himself used: Norse mythology, rich and messy.