Maybe The Little Flowers of St. Francis or the Life of St. Francis by St. Bonaventure. Another good place to start is the OFS Rule and Constitutions. Maybe look up the Friars Minor Rule and the Rule of St. Clare. You can find the writings of St. Francis online, they will be a big help to understanding Francis. Most of this can be found online for no cost. I’m borrowing the Omnibus of Sources and that is a complete work of St. Francis but is old and costly,. Pity they don’t have a newer cheaper version. The book Franciscan Prayer by Illia Deleo(although I’m not a fan of all her work) is good for understanding the Franciscan prayer tradition. It’s hard to find a book directed to Secular Franciscan that is any good in my opinion. There are also Pope’s writings of the Saint which are good. Anyways God Bless you, may Francis walk with you and if it be God’s will may you be professed.