The Book of darkness is a book held in place of a shield, and is the God book aligned with Zaros. It can be purchased from Jossik for 5,000 coins after completion of the Horror from the Deepquest, and previously with The Dig Site quest, but after a recent update, you no longer need it. Ancient books must be made using all four torn Ancient pages which can be purchased from other players, or be obtained from Treasure Trails.
Equipping a book of darkness allows the player to preach and perform ceremonies, such as blessing, last rites, and wedding rites, by operating it (none of which have any effect on the game itself, and are simply recited lines of text), costing 25% of the player's special attack energy.
Lost books of darkness can be reclaimed by speaking to Jossik, complete with all pages that have been added, for no cost. Alternatively, for those who do not wish to travel to the lighthouse, Perdu will return them to players for 14,000 coins.
It is worth noting that the book's pages sound as if they were written recently since the book talks about followers of Zaros being oppressed. However, when Zaros was in power, he had great influence and many followers.
Filling the Book of darkness currently costs 778,411 coins when buying each page individually or 852,067 when buying the item set.