The door into your personal office (not the door into your firm’s space) is almost always in interior, hollow-core door.  Why? because it’s cheaper and people generally only have a door there for privacy.  Nuts!  That door is key to your personal safey.  It should be a solid-core door that is well mounted to the wall with 3″ screws at a minimum.  If you want to be extra safe for extra cost, get a steel door that is fronted with a wood veneer.  Be sure to mount it solidly using 3″ screws or larger.