Controlling the center versus occupying the center - What is the difference? I would like to know if there are any general guidelines which help determine, at what moment to occupy a center square or to just control it? Are there any books or articles that anyone know of which deal with the differences with example?

Also I am trying to understand what is the difference between controlling the center with pawns versus pieces.
Hypermodern theory , or controlling the center from a distance rather than occupying it immediately, came about in the early part of the 20th century as a new way of addressing the old problem of center control.  However, as many knowledgeable people have pointed out, it's easier to try to get something from the start rather than giving it away first and then trying to get it back, and hypermodern openings are not seen that often at higher levels of play now.  In other words, they don't feel that it's as good as the classical approach of contesting the center immediately, and I would certainly suggest that you first become proficient with those openings before attempting to specialize in hypermodern openings. But books from the early hypermodern period by Reti or Nimzowitsch for example will go into detail about that type of play.  I haven't kept pace with what the more modern books on this subject might be. Distant center control is done by fianchettoing the B's on the "b" and "g"  files. Attacking the enemy center pawns is done with the "c" and "f" pawns supported generally by the B's. Or you can control the center before your opponent has occupied it with initial prophylactic "c" pawn (Sicilian Defense) or "f" pawn (Dutch Defense) moves.  Attacking the center this way would have to be done fairly early before development has been completed and it has been too solidly defended.  The rationale is that by diverting/removing the enemy center pawns, you will then be left with the more important center pawns of your own with which to subsequently occupy that area.