On the Internet, I found one copy of Milly's poems available, which I have ordered: a first edition, published in 1924 and titled Unfulfilment, under the pen name "Joan Arden." For this first edition (by which the seller means "only edition"), said to be in excellent condition, I am paying a mere 30 dollars. Apparently there is not a run on this book.
From the bits and pieces quoted in Spurling's biography, I can only say that Milly may be a quiet poet, but her eye and the purity of her diction are notable, as this tiny excerpt shows:
O only once to loose my hold, and slipDown the familiar bank, and feel the chillOf water lapping round my feet, and hearThe sounds of distant music in the wind.
I expect to be disappointed by her book, yet I'm hopeful also. Something about those lines moves me, rather as John Clare can move me, almost in spite of myself.
Really: look at this stanza . . . what could be more beautiful?
And still I see how clearly shines the lightOn winter branches, and how the dripping rainDeepens the colour on the hills, and howTo draw those horses plodding up the lane.