Bookworm is a transformation in The Binding of Isaac, added in the Afterbirth+ DLC.
Obtained by acquiring any 3 items with the book tag.
Causes Isaac to fire an additional tear 25% of the time.
This page shows the latest information as of the new Repentance+ (Online Co-op) DLC.
To see information about this transformation for the previous Repentance version, check this page instead.