The Imitation of Christ
Brevarium Romanum, not simply as a prayer, but the sermons of great Popes, and Early Church Fathers, who explain the Scripture readings for the day or week, as well as the Saint of the day are contained in the reading.
Catholic Evidence Training Outlines, originally a Sheed and Ward book republished a couple years back. A wonderful Apologetics text that is a very comprehensive guide to explaining the Faith. It was originally used to train lay street apologists who spoke at Hyde Park in London and Time Square in Australia. From the 1920’s to the 1950’s they brought thousands into the Church
More to the list later