If I was sending out a very high value book I'd use the customs form E-15 but 99 times out of 100 you'd be fine with the regular export declaration using the B 13 export declaration form - the E-15 form requires a customs inspection prior to export (I'm not sure how well most couriers handle that process)  While these are the customs form options, your need to prove export upon re-entry can usually be accomplished to satisfy Customs by simply retaining a copy of your submission form along with y