What does the cross and the dove on the trilogy covers stand for?
They represent the Huguenot cross. It first came about in 1688 after the religious persecution under Louis 14th of France, which is when the trilogy is set.
Notice how, between the arms of the cross, the four fleur-de-lis or lily flowers—which here signify the Four Gospels—create an empty space beneath them that look like four hearts. Pretty, isn't it? Each flower has three petals which make up 12 in all and are said to symbolize the Twelve Apostles.