I'm not saying I distrust Mary of Agreda but the best source for what happened to Our Lady is Tradition (i.e. What the Church Fathers taught, who were the direct successors of the Apostles).
Mary of Agreda's book does not conflict with tradition. Read the book, the part about her death is very detailed, a quick read, like one chapter long, 20 pages maybe. She never aged past 33 years of age, and all the apostles were called to bid her farewell, she was in perfect condition when she "died" at her request, like her son had died. Then she passed the the other side. One could call it a "dormition", I could understand why, as she chose to die rather than just be assumed alive to heaven. It was her choice, she layed down and died. It was her choice.