I suggest you look into the Philokalia. There are some great editions available in English that were translated by Bishop KALLISTOS (Ware). The entire Philokalia is not yet translated as I understand it, and the translation by Bishop KALLISTOS comes in volumes. Also, I once read a great book called The Art of Prayer, but I can’t remember the Author at the moment.
In regards to the question about mental images while reciting the Efche (Jesus Prayer), we are admonished to not allow ourselves to even have thoughts (images or otherwise) whilie praying the Efche. The Fathers teach us that these images and thoughts come from the devils. But we are not to become troubled by these thoughts/images either. We are simply to let them come, and then let them go. We are not to engage these thoughts or think badly of ourselves because we have them. The image or the thought is simply the beginning stage of having our focus taken off the prayer. We should not try to do spiritual battle with the evil one when these things occur. Rather, we are to simply and calmly bring our mind back to the words of the prayer and the images and thoughts will eventually subside as the Master will send His Holy Angels to fight them off for us! If you have images or thoughts, simply continue with the Efche and these things will pass. Also, I would encourage Latin and Eastern Catholics, and the Orthodox alike to begin praying the Efche. This most holy of all prayers will melt away our ego and allow us to reach unity, because what can cause humility and spiritual enlightenment more than learning to pray in our hearts constantly, thereby recieving the vision of the light of Tabor (or as I think Catholics call it, The Beatific Vision). I would also encourage anyone wanting to learn more about the Jesus Prayer to read the Biography of St. Seraphim of Sarov by Archimandrite Lazarus Moore. It is a great read, and an inspiring spiritual help.
The sinner,
Josh
Lord Jesus Christ, Son of God, have mercy on me a sinner!
Most Holy Mother of God, save us!