Spell Tome

This is the blank book sitting under most of the crafted magic weapons in Hardmode, so you will need more than one. The Wizard has to be found in the caves and rehoused before he sells anything, and he can die, so buy several every time you are standing in front of him instead of one per recipe.

The recipes it feeds run the whole of Hardmode magic, starting with Golden Shower. It also has a Shimmer path into Advanced Combat Techniques: Volume Two, which is reason enough to keep a spare tome around even on a character that never casts anything.