If, when looking down on the head of a book, the corners are not square it is said to be cocked or rolled. Also known as a spine slant. (Note: cocking can also involve a book’s spine being slightly twisted or non-vertical at either end that is not severe enough to cause spine slant.). Frequently the result of stacking books on top of one another.
The top two pictured books are square; the remaining two are cocked with the first one a bit more so.