I don't think it is necessary to add new prayer books to the game, prayer already feels all over the place as it is. Besides, by lore Prayer ain't actually something to do with the gods, Necrovarus's notes explain Prayer is actually harnessing the power of the dead and curses are just a different way of using it.
We have 6 God Books and 4 Elder God Books (1 of which is technically a skiller god book), a Book of Death, and a book from a giant tree boss. I don't think we need any more unless it is intended for skilling.
Could be useful if we could enhance god books to get these effects on top of their current illuminated effects. The spec one would make the sara book competitive with grim as currently it's fully outclassed.