Hi, Kevin —

These books are not historical, despite claims to the contrary. They are works of fiction. They are attempts to push a certain agenda by presenting fiction and legend as established history.

• Opus Dei is a personal "prelature", a voluntary association of priests and lay people.
  Its mission is to spread the message that work and the circumstances of everyday life
  are occasions for growing closer to God, for serving others, and for improving society.
  Its founder, St. Josemaria Escriva, was one of the first in modern times to recover the
  idea that the pursuit of holiness is the vocation of lay people, not just priests and religious. You can find information about them on https://www.opusdei.org. They have been controversial for some because, at least in the past, they have tended to recruit people in a low-key relational fashion, and so have been called "secretive", but there is nothing secret about what they believe or how they are organized.
  
  
• For information on the Knights Templar, see their article on New Advent.
  
  
• I've never heard of the Priory of Sion.
  
  
• As for the Illuminati, I doubt it's possible to get any reliable information on them; they are what you call a canonical example of a secret society which figures into a lot of conspiracy theories.
  
  
• I've never heard of the Order of the Dragon, either.

Eric