Pretty much everyone knows Mission San Xavier del Bac’s nickname — The White Dove of the Desert. What haven’t been as well-known are the stories behind the art inside it. Until now. A new book, “A Gift of Angels: The Art of Mission San Xavier del Bac” (University of Arizona Press, $75), explores, in words and color photos, the beauty of the Spanish colonial mission southwest of Tucson inside and out. Photos of San Xavier c. Edward McCain.