One of our staff members emailed Nintendo to ask if the art book pre-order bonus would be included to those who pre-ordered from the Nintendo online store and this was the response they received:
"Thank you for contacting us about Xenoblade Chronicles. The artbook that you asked about is only being offered as a pre-order bonus through Gamestop, not through Nintendo.
Nintendo of America"
As of right now, it sounds like ONLY Gamestop is getting the Art Book pre-order bonus so if you want the Art Book we suggest pre-ordering from Gamestop.