I recently finished reading this. It changed my perception of Stevens from an aloof obscurantist into a poet of melancholic desire. It's a short book which reveals his harshness, desire, secrecies and perfection of magnitude. He could be harsh with himself, he desired even as a septuagenarian, his secrecies were: using "he" or "she" instead of "I"; burying the emotional heart of a poem in the middle instead of stating it in the beginning or end; placing the context of the poem in his own work as well as his predecessors (particularly Keats); misleading titles; and his allusiveness. The final chapter covers Stevens' handling of the orders of magnitude between body, mind, garments, environment and nature. It illustrates how he reimagined the differences of magnitude between these elements in successive poems, culminating in The River of Rivers in Connecticut (which I happen to cross twice daily on my commute.) Included are some quotes from Stevens' Opus Posthumous, which prompted me to want to check that out, too.
My vote for favorite Vendler sentence in this book is on page 58: "If there is no medium of verbal solubility, perhaps one can only imagine two immiscible liquids with a metonymic impermeability." It seems that every book I've read of hers is usually very clearly written, but has one trademark sentence like that in it. I love it!