One helpful book on the Eastern Church (particularly Eastern Pneumatology) is Stanley M. Burgess' The Holy Spirit: Eastern Christian Traditions published by Hendrickson (1989).
Burgess defines ἐπίκλησις this way:
"Invocation of the Holy Spirit. Such an invocation is common in Eastern Christian services, including during the Eucharist (or Mystery) when the Spirit is invited to make the elements into the Body and Blood of Christ. Some epikleses ask for the descent of the Holy Spirit on the people as well as on the elements. In the Egyptian Anaphora of St. Serapion, however, there is an epiklesis asking for the descent of the Logos instead of the Spirit. The epiklesis comes after the words of institution" (229).
Θέωσις (Théōsis) simply means "deification" or "becoming God-like." Burgess explains that théōsis is "the goal of the Christian life in Eastern Christianity" (232). The soteriological implications of théōsis are also worthy of consideration. Compare 1 John 3:1-2.
Written Friday, August 8, 2003 at 5:31 P.M. by Edgar Foster; slightly edited on October 14, 2013 and edited again on 12/26/14 at 11:50 P.M.