Well, that was unexpected. Like I mentioned last post, I thought black was going for a Nb8->a6->c7 setup to push b5, but apparently that's not the case. If this is a book move then I am most definitely out of theory here and on my own. Time to take a couple of minutes to break down the move.
My gut feeling is that it's too early to put the knight on b4. I'm not exactly sure what it does other than allow black to play a6 next move, which is what I'm thinking he'll play (of course we've seen how good I was at guessing his reply last move). But I need to put aside my initial bias and see if this is actually a good move or not.
On the plus side, it puts the knight on an advanced square, allows black to play a6 to kick the bishop, and keeps pressure on d5 in case white tries to push e5, which is a common idea in the Benoni.
As for negatives, it commits the knight to that square very early where it might not be optimal and pretty much rules out a b5 push which seems to be a good source of counterplay for black in these positions. Also, once black's knight on d7 moves, pressure on e5 will be lessened which could make a later e5 push by white easier to achieve.
Another thing to note is that black isn't putting any pressure on the e4 pawn, which is a key point in white's position. Most other times I've played against the Benoni black tries to put pressure down the e-file, but that isn't happening here. Not sure how that will affect the rest of the game, just an observation.
In general, spots like this are a leak in my game. What I mean is I struggle in positions where I have a vague idea of what I want to do, but there's nothing immediately forcing for either side so it's sort of like this nebulous grey area in my mind.
After much thought, I've decided to play 12.Re1. I feel like this move does several things. It develops the rook to a better square, prepares an e5 push at some point, protects e4, and even opens up the f1 square for a bishop retreat (and potential re-maneuver with Bb5->f1, then g3, then Bg2) if necessary. I also sort of arrived at this move by a process of elimination. I don't want to retreat the b5 bishop without making black spend a tempo on a6, I'm not sure where the queen and c1 bishop will go, and the white knights are nicely placed. I don't want to get caught in the trap of just lazily putting pieces on natural-looking squares (like Re1), but at this point I don't feel like the position is defined enough for me to play other, more committal moves. Could be wrong though if I see a flurry of spoilers