A camera ready hard cover file should include space for 1″ on all four sides (regardless of actual book size), back cover, front cover, and spine allowance. Examples –
5.5×8.5″ with a 1″ spine would be 14×10.5″
6×9″ with a 1″ spine would be 15×11″
8.5×11″ a 1″ spine would be 20×13″
This section is drawn from http://www.gregathcompany.com/hb/color/coverdesign.html