The other reason Wizards of the Coast created the Book subtype could be for strictly flavor reasons
We get three to four full 'Standard' sets annually (about one every three months) and a varying mix of smaller sets - usually focused on specific formats, or revisiting a popular past setting with additional cards
Before Griselbrand, there used to be room for one oddball choice , like Sheoldred for long-game advantage or a Platinum Angel to stop combo decks that don't interact with the battlefield, somewhere in your 75
Creature - Fungus Snake Color tag templates(None) Oversoul of Dusk Protection from blue, from black, and from red Creature - Avatar Spirit Color tag templates(None) Painter's Servant As this creature enters, choose a color
Ive seen posts explaining how to game this system, but Im not tech-savvy enough to know if its real or not