A decade ago Emily Fox Gordon made her debut with “Mockingbird Years: A Life In and Out of Therapy,’’ a memoir developed from an essay included in her fourth and latest book, “Book of Days: Personal Essays.’’This new collection of her work over the intervening years is stunning, not only in the precision and beauty of the language, but also in the author’s willingness to revisit events in her life — even ones she’s already written about — and to change her mind about them. Though each essay stands alone, the book as a whole traces the path of a woman becoming a writer.