There are several books about writing that I re-read on a regular basis: Bird by Bird by Anne Lamott, If You Want to Write by Brenda Ueland, and Writing Down the Bones by Natalie Goldberg. None of them are about the technicalities of writing, but rather about the fulfillment and connection to oneself and the world outside of oneself that writing can bring.
Writing Down the Bones was one of the first books I purchased when I decided to leave the world of writing for a living and to write for myself instead. Churning out articles day-in and day-out had burnt away my soul. If I continued to do it as an occupation, I knew I would never write my novel(s). Goldberg's Zen approach to the writing process was exactly what the doctor ordered.
Now she's endorsing NaNoWriMo.