Let me clear up a little misconception that’s been going around.
Peter Jackson (and his screenwriters, Phillipa Boyens and Fran Walsh) did not invent much of anything. Thorin’s story comes from Return of the King, Appendix A. The story of the Necromancer comes from the last chapter of The Silmarillion. Radagast also appears in The Silmarillion. Stories of Gondolin appear throughout the Silmarillion, as well as in the Lost Tales and various others of Tolkien’s extensive collected writing.
If you’d like to know more about even the tiniest detail in Tolkien’s world, it’s all there for the reading.