Every learned rank of a spell name.
Drops the index; a running build is abandoned.
One spell ID of each spell name in the book (the first one found); empty before the index is built.
A spell was just learned: add it to the ranks of its name. The index is not rebuilt on learning (a full book walk froze the client), so without this a new rank would only show after a /reload.
Rebuilds the index: all at once with isImmediate, otherwise a few skill lines per frame.
The first spellbook ID of a spell name (undefined before the index is built).