To testify is to swear from the root of generation, the word bound to the testicle, the organ once laid hold of when an oath was given, so this is a witness made from the deepest creative center of the body. The prophecy is any sacred writing, not a forecast of coming events, and the book is the book of life, the coded record carried in a person's own inner structure. Adding to these things draws the plagues, the physical afflictions that press a person back toward stillness and the inner work.
The warning is that tampering with the sacred pattern already written in the body brings on the very bodily distresses that will finally drive a person to sit quiet and meditate.
Book: the book of life, the coded record within the body.
Plagues: physical challenges and afflictions that drive a person toward meditation.
Prophecy: any sacred writing, not a prediction of future events.