Which supplement books are the best for additional options for creating droid npcs and players?

I would just go with whichever book covers the career you choose. Beyond that, if you want droid crafting rules or other tech heavy stuff, look at Special Modifications (EotE Technician book), or Fully Operational (AoR Engineer book) if you are willing to wait until it's release at the end of the month. Droids also have a high cybernetics cap and no stigma for using them, so any book with lots of cybernetics is helpful, particularly Lords of Nal Hutta or Special Modifications (which I think even has a few droid-specific cybernetics).