The NML was a series of books commissioned by the School Math Study Group (SMSG). A quick dive into Wikipedia turned up the interesting fact that the SMSG was formed after Sputnik flew over. The US decided we were going to need far more scientists and engineers than we had available. The solution to the problem back then made logical sense: increase the number of math literate students available. Hence, the SMSG gave us the NML, a series of excellent books that: "make available to high school students short expository books on various topics not usually covered in the high school curriculum", and the New Math. If only they'd told us what the New Math was for!