Nimzo Indian Ideas/Main Variations

I am new to playing the nimzo indian but I figured its about time because i want to challenge myself rather than always have transpositions into QGD exchange and semi slav structures as black. Can you guys give me some main opening lines and the ideas. I also am curious why black takes the c3 knight when white plays a3. The doubled c pawns are more beneficial and the bishop pair is also important but in other variations black opts to go back to be7? Why is that so? Why is it important to keep/lose the bishop on f8 

I would either get:

The DVD from Jacob Asgaard (http://www.chessbase-shop.com/en/products/the_nimzoindian_defence-the_easy_way) for an explanation of the defence and the most used variations. 

Or 

https://www.newinchess.com/The_Strategic_Nimzo_Indian-p-969.html

if you want some serious deep study material. 

Perhaps:

https://www.newinchess.com/Winning_Chess_Middlegames-p-922.html is interesting to because i deals in detail with (pawn) structures that arise from the Nimzo.