I finished reading The Hobbit around a week or so ago, and given how ominously the Necromancer was mentioned, I was sure he was being set up to be the main villain (or somebody very important to the plot). I've read up to the part where Gandalf's telling a now 50-year-old Frodo about the lore of Bilbo's ring, and he keeps talking about the Dark Lord Sauron.
Do we know what happened to the Necromancer? Additionally, are there any extra books that go into detail about who the Necromancer is and what evil things he did during his time as an evil tyrant?