The Philokalia is an extensive anthology of writings on prayer by Eastern Orthodox contemplatives. The anthology was first compiled in the 18th century, and a number of editions have been published over the years, in Greek, Russian, and other languages; currently a four volume set is available in English (which does not include the entire anthology; I hope the editors plan on translating the rest!).
If you want a more succinct introduction to The Philokalia, one option — which, unfortunately, is out of print, and secondhand copies are a bit pricey, but perhaps you can find one at a local used bookshop if you’re lucky — is a book called Writings from the Philokalia on Prayer of the Heart. What I love about this particular anthology is that it includes all the texts that are listed in the book The Way of a Pilgrim as the most important for beginners to serious prayer.
Here’s just a tidbit of the kind of wisdom found in this collection:
As the physical eye looks at written letters and receives knowledge from them through the senses; so the mind, when it becomes purified and returns to its original state, looks up to God and receives Divine knowledge from Him. Instead of a book it has the Spirit, instead of a pen, thought and tongue (‘my tongue is the pen’ says the Ps. [xlv. 1]); instead of ink — light. Plunging thought into light, so that thought itself becomes light, the mind, guided by the Spirit, traces words in the pure hearts of those who listen. Then it understands the words: ‘And they shall be all taught of God’ (John vi. 45), and ‘he that teacheth man knowledge’ (Ps. xciv. 10).
— St. Gregory of Sinai, Texts on Commandments and Dogmas 23, in
Prayer, of course, is far more than a mere mental exercise. But even when we meditate or contemplate, prayer involves bringing our thoughts into the presence of God. This idea of plunging thoughts into light — so that they become light — seems to me to be a powerful way to deal with a distracted mind during prayer. If you are having difficulty settling your restless thinking-mind into the silent presence of God, take a minute and envision all your thoughts, restless or distracted they may be, simply plunged into the wordless loving light of the Divine. Plunge your thoughts into light. Let them become light. And then sit in silence in the Divine presence.