[Spoilers Extended] Book Theories and Lore

Finished watching the show a few days ago and I'm itching for more, but I can't bring myself to read the books knowing that the wait for Winds of Winter is indefinite. But I still want to learn about the lore, especially the white walkers, the Three Eyed Raven, Bran, etc which is why I come to you, starved readers, hoping you can answer my questions lol.

Do the books explain why Bran is able to warg, and why the TER sought him out specifically? Why can Valyrian steel kill white walkers?

What interesting bits of lore never made it into the show?

What are the popular theories for WoW?

Have there ever been theories that were confirmed by the show or GRRM?