Zero Doubler
Zero Doubler is a Boss artifact for Books that causes your first 0 Energy card each turn to be played twice.
Clarification
Its sprite changes when it's been used for the turn.
Any card that costs 0 Energy for any reason is eligible for this doubling, regardless of what its base cost is. For example:
- 1. A Discounted card can be doubled by this if it costs 0.
- 2. A Temporary card discounted by Quick Fix can use this.
- 3. A card played by Admin Deploy is eligible regardless of its native cost (but note that Admin Deploy A will itself take the doubling).
- 4. An Exhausting card played with Clean Exhaust in play will be eligible
The card is considered played twice for all intents and purposes, including for Avid Reader's count, and for the Gemini, you will switch Cannons/Missile Bays before the second play of the card.
Note that the doubled effect will still cost Shard, if applicable, for both plays, meaning that you need twice any Shard cost of the card to benefit from the Shard effect twice. This particularly matters for Bloodstone Bolt.
Strategy
Zero Doubler can be an awesome, flexible increase in output each turn if built around properly. Books herself mainly has Crystal Shard to use with this, which is an entirely respectable option, increasing your Shard economy and adding on card draw each turn if you can find one consistently. Should you manage to find Perfect Specimen, you're in for a treat if you can upgrade it to Perfect Specimen A then use this to double it. (She also has Mining Drill and Bloodstone Bolt, which can be reasonable options if upgraded and in the right deck.) It's probably a good idea to look for other 0 cost cards to make sure you can make use of this every turn, such as upgraded Basic cards, cantrips like Solar Breeze, and so on.
There are even some spicy combos available with Max's Admin Deploy and Clean Exhaust and CAT's Discounted and Quick Fix mechanics, if you have the right deck to capitalize on them - enabling you to double high-impact cards that this normally can't double.