If you're looking into Orthodoxy, I would recommend this book with the following warning: This book is not really what a professor of mine would call a, "soup and salad" book. That is, it is not one that you can just buy and skim through; it is not light reading.
That having been said, for people out there like myself, who really want to know why the Holy Orthodox Catholic Apostolic Church considers herself to be just that-- THE Church, then this book is for you.
Lossky spends over half the book laying a foundation on the Eastern understanding of apophaticism (describing God by what He is not), asceticism, the Holy Trinity, uncreated energies of the Godhead, image and likeness, the "economy of the Son" and the "economy of the Holy Spirit," before discussing the goal of Christian mysticism which is theosis or union with God, the Divine Light. To me, the heart of the book is in the chapter on "The Way of Union," but it would be meaningless without the preceding chapters.
Lossky quotes profusely from the great mystical theologians of the Eastern Church, from various epochs and geographic locations to display the inherent unity of thought on mysticism in the Eastern tradition.
Readers who need an introductory work before tackling Lossky might want to try "The Illumined Heart" by Frederica Matthewes-Green, "Beginning to Pray" by Anthony Bloom or "The Art of Prayer" by Igumen Chariton of Valamo.