From the preface:

St. Thomas Aquinas (1225 - 1274), one of the greatest philosophical minds of the church, has left us no formal treatise on mystical theology. Instead, his teachings on this subject have been collected from his various works. No such synthesis has been attempted in this book. Rather the editor has simply taken from the Summa Theologica the treatises on religion, on devotion, prayer, and the contemplative life, and presented them in an English dress. When occasion offered, he has added to each portion appropriate passages from St. Augustine, St. Thomas' master, and more rarely from the Commentary on the Summa by the illustrious Cardinal Cajetan.

This has been done for several reasons. The mystical life is the life of union with God, and it is based essentially on prayer and contemplation. But prayer and contemplation, though simple in themselves, are yet fraught with difficulties and dangers unless we be wisely guided. And as Father Faber wisely says: When we ask for instruction in these things, let us, by all means, make an appeal to those whose names begin with St. - let us, in other words, go to God's saints. And the reason is simple: These Saints are no mere idle signposts who point the way but stand still themselves; they themselves have been where they would have us go; they speak from no mere theoretical knowledge; they themselves have tasted and seen that the Lord is sweet!