Interestingly enough, St. Benedict talks about lectio divina, or prayerful reading, in Chapter 48, The Daily Manual Labor. The monks day was punctuated with The Liturgy of the Hours; lectio divina provided a means for the prayer that was begun in the oratory to continue and the fruit that came out of the time spent in lectio divina could then be further mulled over during times of work -- thus sanctifying the entire day.
Too often we confuse lectio divina with spiritual reading. Both are important, but they are distinctively different. The best book used for lectio divina is the Bible, Old Testament or New. Our aim is to allow God's Word to transform our hearts.
Spiritual reading encompasses many different categories, from books on the saints, writings of the saints, theology, Christology, church history, etc., and while the text we read impacts us, can change us, it doesn't put us in touch with God's own Word. Only Sacred Scripture can do that.