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 information as of the latest Repentance release.
To see latest up-to date information about this transformation for the new Repentance+ (Online Co-op) DLC, check this page instead.