Its a special book held by some Snake Cult troops and can be sometimes found in the loot. They are useless until the players reaches 500 renown and joins a KO. You can trade these books with the knight of the KO you belong for more denars, relation with the order and rank points. They can be dropped by:
Each will award you 1000 denars, 1 relation point and 3 rank points with your order when handing them to the knight of the order you joined.
Note: Since 3.9, you can also hand in these books to any knight which you have negative relations with his order. For every book, it will raise relations with his order by 1 and give you 50 denars. But if you bring him 3 books (no matter if they are Heretic Writings, Snake Cult Rituals or mixed), you will get instead of 3 relation gain, 5. However, you will still gain 150 denars (50x3).
Note: Since 3.9, defeating an Inquisition Army will always reward you with one Snake Cult Rituals book. Defeating an Snake Cult Army will always reward you with two Snake Cult Rituals books.