Sometimes I'll have a student or friend ask me to look over their writing, and especially if they're looking for encouragement rather than specific feedback, I'll recommend them further reading. These are the books that I often recommend and that help me understand my writing and writing process.
Writing Down the Bones: Freeing the Writer Within by Natalie Goldberg & Bird by Bird: Some Instructions on Writing and Life by Annie Lamott
For Goldberg and Lamott, writing is an exercise, not an assignment, in which we can grow, learn, and understand.
Triggering Town: Lectures and Essays on Poetry and Writing by Richard Hugo & Nine Gates: Entering the Mind of Poetry by Jane Hirschfield
I return to the ideas of Hugo and Hirschfield over and over. Both focus on writing poetry in particular, and what they say about it is so true to my experience that I've read both of these books cover-to-cover and reread them in part frequently. My undergraduate students seem to prefer Hugo's more direct tone over Hirschfield's philosophical one, but I find them both equally valuable.
Ron Carlson Writes a Story by Ron Carlson & From Where You Dream by Robert Olen Butler
These books are both written for fiction writers, and I first encountered them in Neil Connelly's Form and Theory of Fiction Class at McNeese State University. However, fiction writer or not, I have a better understand of my poems' speakers (and my writer-self) because of Butler, and I need Carlson to remind me to stop looking up words in the dictionary and to keep writing.
Poetic Form and Poetic Meter by Paul Fussell & The Art of Syntax: Rhythm of Thought, Rhythm of Song by Ellen Bryant Voigt
I learned everything I know about writing well on a sentence- or line-level from Fussell and Voigt. These are not for writers who are "faint of heart" but for those who really love words.