This is the book for which St. Louis Marie de Montfort is probably best-known. It should, however be read in the context of The Love of Eternal Wisdom, where he makes it plain that a "tender devotion to the Blessed Virgin" is only a means (although the most effective means) to acquire and preserve Divine Wisdom.
In the True Devotion, St. Louis Marie sets out his teaching on devotion to Mary in general, and proposes a particular form of devotion, involving a total dedication or consecration of oneself to Jesus through the hands of Mary. The first part of the book is devoted to showing that devotion to Mary is not an end in itself. It is always a means to becoming more dedicated to the service of Jesus Christ. It is, however, he claims, a necessary means to this, and is indeed the surest way to achieving this goal. He examines the qualities of what he calls "true" (or genuine) devotion to Mary, as opposed to false devotion. And he makes it clear that there can be many different kinds of "true" devotion to her. Yet he claims, based on his own experience and reading, to have found one form of devotion to Mary which achieves its aim (to bring us closer to Jesus Christ) more effectively than any other.
The form of devotion to Mary to which he refers (and which he earnestly proposes to his readers) consists in a total dedication of oneself to Jesus Christ through the hands of Mary. This total dedication he calls "consecration", and he is careful to explain that, even though we may speak of "consecration to Mary", this must always be understood as only a step on the way to "consecration to Jesus Christ". The rest of the book is taken up with an explanation of what this total dedication means in practice, and with descriptions of its effects in a person who undertakes it, intended to encourage us to embrace it. It also examines various "practices" of devotion intended to help us to live it out, both "exterior" and "interior" practices, as he calls them. Among the exterior practices, he speaks of the recitation of the Rosary, a topic which he treats more fully in The Admirable Secret of the Rosary.
This form of devotion to Mary was known in St Louis Marie's day (and before) as "Holy Slavery", and he spends some time explaining the meaning of this phrase, insisting that, far from being a slavery of compulsion, it must be a "slavery of love". Other phrases, more in tune with our modern way of thinking, could easily be used in place of this.
Another of St Louis Marie's works, The Secret of Mary, is a shorter presentation of much the same matter as is found in the True Devotion.