Including 3rd party materials, other than Cohorts and Companions, what books contain information for fleshing out usage of the Leadership Feat ?
Main 3PP book I know is the Leadership Handbook, which also is contained in the Ultimate Charisma (though don't know how much was expanded).
For pre-Pathfinder there's Power of Faerun, though I'm mainly looking for recent Pathfinder materials.
Ultimate Campaign.