From "The Notebooks of Robert Frost," edited by Robert Faggen, published this past January by Harvard University Press.

Here's the intro and some of the excerpts as printed in Harper's:

[the book is comprised of] fragments culled from more than forty notebooks Frost left behind, containing writings dating from the 1890s to shortly before his death, in 1963.

"A sentence carries a certain number of words and those have their sound, but the sentence has a sound of its own apart from the words, which is the sentence proper. It was before words were. It still has existence without the embodiment of words in the cries of our nature."

"In composing poetry I am packing up to go a long way on wings."

"...Story of planned economy on Easter Island where the population was limited to nine hundred by killing either the newborn at one end or an old person at the other.
Story of the hard drinker's disbelief in disinterestedness.
Story of the man who originated the slogan, No rivers to the sea..."

"If an idea for a poem comes to me when I am playing golf at the club, I just tell it to trot along for the present and come round to my office tomorrow morning. You come round to my office at eleven o'clock tomorrow morning and I'll write you down, you little rudiment, so you'll think you were always written down. Call a man of business in his hours of business."

"Crash. There goes another young ideal."

"Adventurous is not experimental. Experiment belongs to the laboratory. Adventure to life. Much of recent art has been merely experimental. It tries poetry with first one element and then another omitted. It leaves out the head. Then it is too emotional. It leaves out the heart. Then it is too intellecutal. It leaves out the feet. Then it is free verse. Adventure ends in the poorhouse. Experiment in the madhouse. Water spout theory of learning from above down from below up till it meets."

"...An artist delights in roughness for what he can do to it. He's the brute who can knock the corners off the marble block. So also is the statesman politician: only the statesman works in a more protean mass of material that hardly holds any shape long enough for the craftsman to point it out and get credit for it. His material is a rolling mob. The poet's material is words that for all we say and feel against them are more manageable than men. You get a few words alone in a room and with plenty of time on your hands you can do almost your will on them."

"I had these higher thoughts long before I had to have them as a refuge in trouble."