In the preface, DeSales is using an analogy by comparing himself with Glycera. He is using this to contrast his proposal for a devout Christian life and the proposal of others before him. What is the difference in their proposals for a devout life?
His predecessors were targeting people who have given up their lives to God. These are priests, monks, religious (nuns, etc). DeSales, however, is addressing his recommendations to common people like us. This is where he says each boquet uses the same set of flowers. They can be arranged in so many different ways to give a different look. To him, flowers are Catholic doctrines. The Holy Spirit can rearrange these flowers to make the doctrines look different but they are the same doctrines. So, De Sales is saying that he is presenting the same doctrines in a different way suitable to lay people.
You are right mostly lay people, his usage of retirement makes me think may be he is talking about people who are detached from every day worldly chaos and are in to a monastery life. He says he is going to instruct to people leading life in the turmoil of worldly affairs how to live a devote life in the midst of this. He uses an example of mother-of-pearl oyster for this.
That is correct jails. He uses the "flower arrangement" analogy to describe is his approach. A set of flowers can be arranged in many forms as bouquets, but the same flowers. Same doctrines (flowers) can be presented in different ways (his approach vs. his predecessors' approach). He presents in such a way that lay people can understand what he says.
