Middle of book 3.
I don't know...I think you may have to deal with overlap or you're going to be confused by storylines that HBO merged up.
Middle of book 3 is where the red wedding takes place. You could start there and be current, but there are going to be a lot of characters that you've not been properly introduced to.
Book 1 was almost chapter by chapter the same as season one.
Book 2 HBO compressed a lot of stuff. I think I'd start with Book 2, knowing that you're in territory you've already covered but picking up depth and details that inform what is going on in the later books.