The decks that we will consider are
- True Draco Invoked
- True King Yang Zing Dino
Target 1 face-up monster on the field; change that target to face-down Defense Position.
A simple, short and sweet effect.
Book of Moon is very splashable and has seen a Limtitation and Semi-Limitation before. In earlier days when the cardpool was still shallow, Book of Moon provided utility like no other cards could. Being a Quick-Play spell it could be activated on either player's turn, firmly rooting its position in many decks.
Over the years, Book of Moon became lesser used as more and more cards are released, with some providing a better advantage or usage due to the metagame at that point of time.
The rise of monsters that had "Cannot be targeted" protection also contributed to Book of Moon's fall from grace. During the July 2016 OCG Format, when ABC, Majespecter Metalfoes and Blue-Eyes occupied the top tables, Book of Moon was casted aside in favour of Book of Eclipse.
The counterpart Book of Eclipse had a larger range of targets, being capable of flipping all of the opponent's monsters down with a non-targeting effect. Whereas Book of Moon could only flip one foe face-down and had to be able to target it. During that metagame populated with Majespecter Unicorn - Kirin and Azure-Eyes White Dragon, Book of Moon was definitely far less from ideal.
However, with the fall of Metalfoes and its predator - Blue-Eyes, a year later, we might start to appreciate Book of Moon once more.
True Draco Invoked
One argument for playing Book of Moon in a True Draco deck is the synergy between Dragonic Diagram and Quick-Play spells. Players previously utilised Cosmic Cyclone, The Monarch's Stormforth and Forbidden Chalice to achieve this, so Book of Moon may not be such a farfetched idea after all. However, the problem remains that Dragonic Diagram is limited, which would bring me to the next point - The Invoked Engine.
Aleister the Invoker has a mediocre ATK at 1000ATK points but its defense stats are quite reputable (1800DEF). Furthermore, Aleister the Invoker gains a search when it is flipped face-up too.
Now might also be an appropriate time to mention how Castel the Skyblaster Muskeeter saved my life once when I used its first effect (yes it actually has one "Detach one to flip a monster face-down" effect in case your forgotten about it) on my Aleister the Invoker, eventually Flip Summoning it to gain a search.
My argument is that Artifact Lancea and Dimensional Barrier is gaining traction. Some players go as far as to play Artifact Lancea in the Main Deck. Hence, I feel that preparations and failsafes have to be in place when things go south and Invocation gets interupted. For other Fusion-based decks, having a Fusion Summoning disrupted might not be the end of the world but if the opponent manages to send Invocation to the graveyard and have Aleister the Invoker join it soon, then the entire recurring engine is disrupted.
Book of Moon comes into play here, when Alesiter the Invoker is targeted for an attack the following turn, allowing you to retreive another copy of Invocation.
True King Yang Zing Dino
Yang Zing monsters are another reason to play Book of Moon. In the 'Double-Negation' play performed by Denglong, First of the Yang Zing, Chiwen and Bi'an forms Herald of the Arc Light. This second Synchro Summon during your turn, can be easily disrupted by flipping either one face down. Denglong, First of the Yang Zing also only Special Summons a Wyrm monster when it leaves the field face-up.
Book of Moon would allow you to take care of it with relative ease, without the need to worry about its trigger effect.
Against ABC decks, I have nothing much to add, except that it would be worth noting that Monsters on the field can't be sent for Link-Summoning when they are face-down. Furthermore, ABC decks currently rely heavily on Destrudo, the Dead Drake of Dread. Synchro Summons can be obstructed by turning either the non-tuner or tuner face-down. It would also pose problems for the opponent when a Union Monster targeted by Union Hangar gets flipped face-down, disallowing it from getting an equipped Union Monster.
I personally have high hopes for Book of Moon. Although I am not so optimistic to label it as meta-breaking, I will definitely give it a second thought at 1-2 copies during this format.
Flipping an opponent monster to face-down defense position means that it will be useless in the face of Link Monsters. But currently, the metagame has yet to be dominated completely by Link Monsters.