The Secret of Mary presents much the same matter as the True Devotion to the Blessed Virgin, in a shortened form, but it also includes a section on "The Tree of Life". It seems to have been written for a member (or members) of a Religious Congregation. The title indicates the way in which St Louis Marie presents his topic: he wishes to disclose a "secret" of holiness, or a "secret" for finding happiness, and that "secret" is to be found in Mary, and in particular in the form of devotion to her which he proposes in the book.
As in the True Devotion, the first part of the book is concerned with the "necessity" of a genuine devotion to Mary, the Mother of God, for a real knowledge of, and adhesion to Jesus Christ. Again, this devotion to Mary is a means to this end, never an end in itself, and is necessary only because God himself has chosen the way of Mary to reveal himself in Jesus Christ.
In the second part of the book, St Louis Marie examines briefly some genuine forms of devotion to Mary, before presenting what he calls "the perfect practice of devotion to Mary", which he says is "unknown to many and practiced by very few". This perfect devotion, he says, consists in "surrendering oneself in the manner of a slave to Mary, and to Jesus through Mary, and then performing all our actions with Mary, in Mary, through Mary and for Mary", referring to this as a "consecration". He goes on to discuss what this means, and in particular what he means by performing all our actions "with Mary, in Mary, through Mary and for Mary".
In a kind of supplement at the end of the book, we find two very beautiful prayers: a Prayer to Jesus, and a Prayer to Mary; and a short metaphor for this form of devotion, which he calls "The Tree of Life".
The matter presented briefly in the Secret of Mary is much more developed in the True Devotion to the Blessed Virgin.