Having some vague idea of its nature and purpose, people undertake the Kriya discipline for their own reasons.   While getting along with the practice, they refine their approach through any kind of readings, both properly related to it and improperly tied with esotericism, New Age thought and alternative medicine.   
I am one among thousands of people who shares a great enthusiasm and interest in the Kriya discipline.   Since the last few years, being frustrated with the polemics among different Kriya schools - which reveals mercilessly and limpidly that Kriya is taught with many deformations and cuts - I have created this website where I share all I know about this subject.   The purpose is to spare other researchers time, delusions, anger and frustration.   (Of course one must have a self-teaching quality. Otherwise one can receive all the best educational material about Kriya and personal initiation by a renowned Kriya Acharya, but will leave everything in a few months or years.) 
   My decision to bring out my knowledge and information about Kriya has subsequently brought me to interact with many people.   This interaction with a variety of people has brought me great joy, and has given me the impression and encouragement of living my life more intensely.   In this manner I have received many incentives to deepen my knowledge of Kriya and enrich the book continuously.
What bewilders me is that some people write to me after only having quickly browsed one or two pages of the book.   They ask me questions that I have already provided precise answers for in the book, substantiated with information and opinions about matters for which I have devoted pages and pages.   (Just to give an example, some ask: «What were your experiences with Kriya?» as if to say that the first and the third parts of the book contained the ravings of a lunatic.)
I have trouble with putting myself in their shoes; I sincerely don't understand their intentions.
   However, I do understand this inconvenient question could be solved by answering: «Please go to page such and such» or, more kindly, by copying part of the book in my reply.   The problem is when they ask an opinion about delicate matters, which may concern a particular organization of Kriya, a famous teacher, a book, a theory… they start a debate about issues on which I have already carefully expressed my opinion in the book but in a general way, obliquely, in order to avoid taking sides.   There are pages from which, using the faculty of reason, my point of view can be easily guessed. 
   Nevertheless, I answer sincerely but shortly to their query.   Perceiving my remarks to be very synthetic, they don't understand their grounds and implications and react surprised, reproaching me of wearing blinders and being rash in my reply and judgment.   If after this crosstalk they would stop writing, the problem dissolves into oblivion leaving only a trace of bitterness - the worry of not having acted in the best of ways.   However, despite some breaks, the exchange of sharp affirmations which are very far from hitting the target continues.   In this manner, the exchange of emails is like playing a game of chess at a distance, where the pawns are prejudices and assumptions.
    In the end a lot of time is lost and wasted.
To avoid all this, the following FAQ serves to reply to the most immediate and urgent questions that come spontaneously to one who has stumbled upon this website.   In my opinion, a researcher should also perform his own work in order to guess what Lahiri Mahasaya's original Kriya was (I don't mean just the techniques, but the essence of it) and get the picture of what the Kriya taught by today's sources is.   It goes without saying that if one has a good guide one can practice Kriya without any further study, but those who stand on their own feet have no other choice than to explore the subject at the full.   I recommend the study of the great religions and of the various mystical movements flourished around them - because Kriya sprang from them. Internet furnishes a boundless documentation.   In my book and in the link pages of my website there are references to some texts that are real jewels, sources of great inspiration.   (Of course those who think that Kriya is an "ancient science received directly from the Divine, available only for the select few, kept as a closed secret by the ancient sages..." and believe similar fabrications.... are not ready to carry on any meaningful search.   Most likely, they will go on indefinitely being occupied with controversy about which is the unique authorized source of authentic Kriya.)   It is clear that no single serious study will help one to find the Truth.   It just helps one to understand the basis of Kriya - from what preexistent practices it has been built - and to understand the personalities who have deformed it according to their cultural and religious conditionings.   To discern this, it is simply not sufficient to only read an article in Wikipedia.   When one attends a University course, one is not surprised at the necessity of devoting one semester of study to one single matter.   Why should Kriya be different and consist of only some vague ideas, visceral perceptions, impressions and hysteria? Discovering, learning and performing Kriya is no picnic in the park, it requires a lot of study and application.   It is a very delicate art.  If you embark on Kriya full of enthusiasm after having read just one book about Sri Ramakrishna, then you will expect that your teacher is a Ramakrishna and you to will become a Ramakrishna.   But you will be awfully disappointed.   Too much exaltation or too much dejection are two sides of the same coin.   Kriya requires balance.  After good reading, a good practice must follow, carried on at the best of one's abilities.   Those who have this quality, will go ahead and receive very great satisfaction, no obstacle will stop them.
In Kriya many details become clear with time and by applying different techniques.   One can become self-realized, a Master, by working in this way.   What I appreciate is the courage of never ceasing to aspire to perfection.
(I counsel to watch the video of Karajan's rehearsal of Schumann's Symphony #4 on Youtube: this video is in my opinion more inspiring and more useful than many so-called spiritual inspiring new age videos).   After good study and proficient practice, there is no danger of useless discussions.
One actually experiences that the exchange of personal reports, of remarks, notes and of any kind of useful material is greatly enriching in advancing their study and practice.