Well, here are a few suggestions. For an overview of Orthodox mysticism in general there is the anthology Roots of Christian Mysticism by Olivier Clement and Theodore Berkeley. Regarding inner calmness, there is the anthology The Art of Prayer by Igumen Chariton of Valamo. Regarding hesychasm, there are any number of books on that topic if you search "hesychasm" or "Gregory Palamas" at a place like Amazon.com. For instance there is St. Gregory Palamas and Orthodox Spirituality by Fr. John Meyendorff. And then there is, of course, The Philokalia, which many have found to be very helpful.