Ok. Open applications/books/modules/front/books/view.php and find:
$page = \IPS\Http\Url::external( $buyUrl )->request( 3 )->get();
if( $page->httpResponseCode == 200 )
{
$showBuyButton = TRUE;
}
You can increase the 3 there or simply remove it all and leave only $showBuyButton = TRUE;. That will make the button always to show up, even if the link is not accessible.