How many books are you sending down and with what method.
The main issue is if you declare over $2500 for the books, customs may ask you for stuff such as the recipient's EIN number (US tax number) and some people don't want to give it.
As I ship FedEx to ComicLink and I include their EIN in the paperwork when I ship, there's no issue, as I've filled out the customs form many times.
But like the other person said, if you choose to declare a value under $2500, there really shouldn't be a delay but your mileage may vary with respect to issues such as insurance and coverage.