When I considered this question, I had to broaden my thinking to consider all books I use to help me create my work--or create in general. Lately, The Creative License by Danny Gregory has been my go-to book for encouragement and inspiration (I even took it with me on my trip to Newfoundland this summer). When it comes to helping me in other ways (choosing colors, layout, style, etc.) I have used my back issues of Print's Regional Design Annuals, as well as old illustration annuals or source books (The Black Book, Directory of Illustration, etc.) to see how other illustrators have handled various aspects pertaining to an image.