St. Francis de Sales, Introduction to the Devout LIfe
This book is about how to live a Catholic life practically. It focuses on the practical application of virtue and how to grow in it.
The only draw back would be finding a good unabridged translation. It was taken up by protestants as soon as it was written and edited heavily,. The Ryan translation is good, if a little dense at times. I’ve also found a translation by Fr. John-Julian, an Episcopal monk, to be very, very good.