turtle back
n.— «I have seen books labeled field guides that are too big or heavy to be comfortably carried on a hike. The guideline is that the book be less than 5 inches wide, 8 inches high and about 1 inch thick so it will fit in a jacket or pants pocket. More importantly, the cover should be some sort of soft material, such as paper, viny, or what is called a turtle back, so the book is a little flexible.» —“Field guides make great gifts for outdoorsy types” by John Ross Tribune-Review (Pittsburgh, Pennsylvania) Nov. 26, 2006. (source: Double-Tongued Dictionary)