Interior Prayer - A Carthusian Guide Book
If you have a sincere interest in a deep and meaningful prayer life this is a very valuable text. Written by "A Carthusian" monk it follows discussions over a period of years given to a group of young monks. It is thought by many to be written by Father Augustin Guillerand (1877-1945) but not wanting to draw attention to themselves the Carthusians often work anonymously.
While this written for and by monks this book is useful to anyone. It is written in very plain language considering the depth and significance of the topics discussed and you will not get lost on lofty theological precepts. It talks through why to pray, types of prayer (intercession, petition, etc.).
Personally I come to Christianity with some Buddhist training. Within Buddhism meditation and contemplative practices are the focus rather than the option of the religious life. And the practices are basically the same for everyone to varying degrees, not just monks. As you experience life on this level many things can happen that require a teacher to guide you. As the contemplative dimension of Christian practitioners is often not shared with their busy diocesan priest or local minister, many people can get lost in this type of work. It is very easy to fool yourself into thinking that you are deepening your spirituality with your contemplative work when you are simply wasting time. It is also very likely as you practice to have a number of issues (visions, mental distortions, etc.) come up while you deepen your practice that you may need help with. While the entire book is valuable this is probably the only text I have read that discusses these issues in detail and provides both solutions and comfort to these events along the way. For this reason alone the book is imperative reading in my opinion.