Given that Bloomburrow feels like a more accessible set than, say, Outlaws of Thunder Junction, it stands to reason that itll be getting its own dedicated Starter Kit, which will help players learn how to play with two pre-constructed, full 60 card MTG decks (and useful accessories too!)
The other reason Wizards of the Coast created the Book subtype could be for strictly flavor reasons
Sometimes it overtakes other duals, as these fluctuate
Seedguide Ash (671 decks, 1%) Every time I cast this card at an event, people read it and say I need to get one of these, especially when they realize you can get dual lands or shocklands with it
At face value, the double Squires should survive combat