The most essential (and most neglected form today) of private prayer is mental prayer. We must pray every day, and though the Rosary and the Sacred Liturgy are extremely important they are nothing without mental prayer. If we pray our Rosary and Breviary well and we are attentive at Holy Mass we ought to be led to a deep and fruitful mediation and the contemplation of the sacred mysteries of the faith. And the fruit we derive from our mental prayer will in turn make our prayer of the Holy Rosary and the Sacred Liturgy all the more fruitful.
First I would recommend reading this wonderful treatment of what mental prayer is: Mental Prayer.
And for further reading I would highly recommend the book: The Ways of Mental Prayer, by the Rt. Rev. Abbot Dom Vitalis Lehodey, Abbot of Bricquebec, O.C.R.; which you can purchase HERE and obtain a FREE digital copy HERE.
And finally, the greatest book on the spiritual life: The Three Ages of the Interior Life, by the late great Thomist the Rev. Father Reginald Garrigou Lagrange, O.P.; which you can purchase HERE and read for FREE online HERE.
And I would also highly recommend the following sermons on prayer:
Mental Prayer is Essential for Salvation
Prayer – Part I – The Quality of Our Prayers
Prayer – Part II – The Importance of Mental Prayer
Prayer – Part III – The Spirit of Recollection
Prayer – Part IV – Difficulties in Mental Prayer
Prayer – Part V – Thanksgiving After Mass: How Long And Why
Other pages on Mediation also on this website: