Publishing terminology defines spine lean as the degree to which a book’s spine deviates from being perpendicular to its covers. This term is frequently applied when discussing used books; those read multiple times likely show some degree of spine lean. Though not necessarily considered an error or defect, severe cases may affect book values significantly.
Spine lean is caused by various factors. Reading a book multiple times and the pressure caused by holding it open can gradually bend its spine over time, while another common source could be storing the book in an environment not conducive to long-term preservation, such as a humid basement or sunny windowsill. Poorly made books will likely succumb to spine lean due to weakly adhered spines.
A book’s spine lean can typically be described in terms of how far away from vertical it has veered; for instance, one that leans 10 degrees would be considered to have a 10-degree spine lean.
Spine lean is essential to both books and publishing; it enables books to be stored neatly on shelves, making it easier for readers to select their book of interest from among a selection. Furthermore, the spine lean facilitates tracking inventory levels more efficiently for publishers while shipping products directly to retailers.