Read "My System" by Nimzowitzsch for why this is so popular. This is known as the hypermodern Indian opening and could lead into one of several other openings in the "Indian" family - Nimzo, QID, KID, Grunfeld
1. d4 Nf6
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^ Focuses on the Ruy Lopez and Grunfeld as Black
Grunfeld is a very theoretical but also very powerful opening.
I can personally say that Kaufman's writing is top-notch, having read one of his books myself.
1.d4 Nf6 is not only one opening as you 've already mentioned ! That's why there is not only one setup, but a lot different ones ! Every opening has its own setup ! You have to opt for which of all those 1...Nf6 openings you want to play, and then look for ideas in this opening !! The openings that are the most popular are: The Grunfeld, The Nimzo-Indian, The King's indian and The queen's Indian ! 
I have almost universally played in my openings 1.e4 e5 and 1. d4 d5. Recently though, I've seen how common Nf6 is to d4 at master level and I've given it a whirl. Its resulted in some pretty interesting and, for me, different types of positions and games.
Unfortunately, I know almost nothing about this 'new' type of opening and some low key research shows me it leads to all sorts of different nimzos and Indians and gruenfelds... Can anyone give me any advice on where is good to go from 1... Nf6 as black, and what some of the ideas are, or where I could look to find out? Why is this opening so popular at Master level?